On 19 February INGV summarized Etna’s fissure eruption that occurred high on the SE flank during 24-27 December 2018, noting that 3-4 million cubic meters of lava erupted and covered an area of 1 square kilometer. After the event seismicity gradually decreased. The last significant event was a ML 4.1 recorded on 8 January 2019; afterwards seismicity was characterized as frequent events with modest magnitudes. Since the beginning of January ash emissions intermittently rose mainly from Northeast Crater (NEC) and more sporadically from Bocca Nuova. News sources noted that the Catania Airport (Aeroporto di Catania – Sicilia) was closed during 26-27 January. Preliminary assessments of some of the ash deposits showed they contained no juvenile material. During 11-17 February ash emissions of variable intensity rose from NEC and were notable on 14 and 18 February. Volcanic tremor amplitude did not significantly vary compared to the previous week, having average values overall. The Catania Airport announced the partial closure of airspace and flight delays during 17-18 February due to ash emissions.

Degassing continues, accompanied by intermittent ash emissions and small Strombolian explosions in June and July 2018

Etna is the tallest active volcano in continental Europe with persistent activity at multiple summit craters and vents. The active craters are Bocca Nuova and Voragine within the Central Crater, the Northeast Crater, Southeast Crater, and the New Southeast Crater (figure 217). This report summarizes activity from April to July 2018 and is based on reports by the Istituto Nazionale di Geofisica e Vulcanologia (INGV).

Figure 217. The active summit craters of Etna volcano: the Bocca Nuova and Voragine craters that occupy the older Central Crater, the Northeast Crater (Cratere di Nord-Est), Southeast Crater (Cratere di Sud-Est), and the New Southeast Crater (Nuovo Cratere di Sud-Est). The years given in parentheses indicate when the craters formed. Photo by Marco Neri, courtesy of INGV (19 July 2018 blog).

Activity through April was characterized by degassing at the summit craters (figure 218), with modest ash emissions from the New Southeast Crater and Northeast Crater in the first week, and occasional small ash emissions at the end of the month. Reduced activity dominated by degassing continued into May with modest ash emission from the Southeast and Northeast craters during the second week, and isolated ash emissions from the Northeast Crater in the second half of the month continuing into June.

Figure 218. Degassing at the Bocca Nuova crater at the summit of Etna in late April. The top image is a photograph of the crater with the location of the bottom image, which is a thermal image showing the degassing and temperature at the vent reaching over 400°C. Courtesy of INGV (Weekly report No. 18/2018 for 24 to 30 April 2018, issued on 2 May 2018).

Throughout June the activity consisted of degassing at the summit craters with isolated diffuse ash emission from Northeast Crater (figure 219). This continued through to July until low-energy Strombolian activity commenced in the Bocca Nuova (from two vents) and Northeast craters (figures 220 and 221). The Strombolian explosions were small, lasting up to several tens of seconds, and were sometimes accompanied by red-brown ash emission. The ejected material was confined to within the craters. More energetic bursts were visible from the INGV surveillance camera located in Milo.

Figure 219. Photos of isolated dilute red-brown ash emissions from the Etna Northeast Crater on the 6 and 8 June. Courtesy of INGV (Report No. 24/2018 for the period 4 to 10 June 2018, issued on 12 June 2018).

Figure 220. A sequence of thermal infrared images of a Strombolian explosion at the Etna Bocca Nuova crater on 17 July 2018. Two vents are active (A and B), with vent B ejecting lava up to a few tens of meters above the vent. The color scale on the right of the images indicates the temperature in Celsius. Images taken by Giuseppe Salerno, courtesy of INGV (24 July 2018 INGV blog).

Figure 221. Photos of Strombolian explosions at the base of the Etna Northeast Crater on 20 and 21 July 2018. The explosions occur when gas pockets burst and eject incandescent fluid lava above the vent. Photo by Michele Mammino, courtesy of INGV (24 July 2018 blog).

On 19 February INGV summarized Etna’s fissure eruption that occurred high on the SE flank during 24-27 December 2018, noting that 3-4 million cubic meters of lava erupted and covered an area of 1 square kilometer. After the event seismicity gradually decreased. The last significant event was a ML 4.1 recorded on 8 January 2019; afterwards seismicity was characterized as frequent events with modest magnitudes. Since the beginning of January ash emissions intermittently rose mainly from Northeast Crater (NEC) and more sporadically from Bocca Nuova. News sources noted that the Catania Airport (Aeroporto di Catania – Sicilia) was closed during 26-27 January. Preliminary assessments of some of the ash deposits showed they contained no juvenile material. During 11-17 February ash emissions of variable intensity rose from NEC and were notable on 14 and 18 February. Volcanic tremor amplitude did not significantly vary compared to the previous week, having average values overall. The Catania Airport announced the partial closure of airspace and flight delays during 17-18 February due to ash emissions.

INGV reported that during 6-10 February webcams at Etna recorded gas emissions from New Southeast Crater (NSEC) and Voragine Crater, and occasional ash clouds from Northeast Crater (NEC); inclement weather prevented observations during 4-5 February. From the beginning of the afternoon on 6 February discrete ash emissions rose from NEC. The ash emissions were continuous from about 2215 on 7 February to 1015 on 8 February, producing relatively diffuse ash plumes that drifted S and then SE. Ash fell in Nicolosi (14 km S) and Pedara (15 km SSE). A diffuse ash plume drifted SW on 9 February.

INGV reported that during 3-9 December activity at Etna was characterized by gas emissions at the summit craters, with periodic Strombolian activity from vents in Bocca Nuova, Northeast Crater (NEC), and New Southeast Crater (NSEC). Strombolian explosions at the cone in NSEC became more frequent on 4 December. In addition, lava effusion became continuous with small overlapping flows traveling about 500 m down the E flank of the cone. Incandescent blocks generated by the lava flows rolled to the base of the cone, and occasional small collapses produced minor ash plumes. Strombolian activity and occasional ash emissions were characteristic of vents in the W part of Bocca Nuova’s (BN-1) crater floor. Gas emissions at Voragine Crater continued from a vent on the E rim of the crater, and Strombolian explosions were evident at NEC.

INGV reported that during 29 October-4 November activity at Etna was characterized by gas emissions at the summit craters, with periodic Strombolian activity from vents in Bocca Nuova, Northeast Crater (NEC), SE Crater (SEC), and New Southeast Crater (NSEC). Strombolian explosions at NSEC were interspersed with long pauses from a few minutes to a few hours. The explosions sometimes produced ash emissions that quickly dispersed; ashfall was deposited around the crater and in the Valle del Bove. Strombolian activity and gas emissions were characteristic of the N vent in the W part of Bocca Nuova’s (BN-1) crater floor. Spattering from the southernmost vent was also visible, as well as gas emissions. Gas emissions at Voragine Crater from a vent on the E rim of the crater were less intense compared to previous months. NEC activity was characterized by Strombolian explosions sometimes accompanied by minor ash emissions.

INGV reported that during 15-21 October activity at Etna was characterized by gas emissions at the summit craters, with periodic Strombolian activity from vents in Bocca Nuova, Northeast Crater (NEC), Southeast Crater (SEC), and New Southeast Crater (NSEC). Strombolian activity at the N vent in the W part of Bocca Nuova’s (BN-1) crater floor ejected incandescent material higher that the crater rim. Spattering from the southernmost vent was also visible. Gas emissions increased at Voragine Crater from a vent that formed on 7 August 2016 on the E rim of the crater, and the crater continued to gradually widen and deepen. NEC activity was characterized by gas emissions and explosive activity of variable frequency and intensity. Fumarolic plumes rose from the rim and crater walls of NSEC and SEC. The E vent in NSEC produced Strombolian explosions and ash emissions which rapidly dispersed.

INGV reported that during 24-30 September activity at Etna was characterized by gas emissions at the summit craters, with periodic Strombolian activity from vents in Bocca Nuova, Northeast Crater (NEC), and New Southeast Crater (NSEC). Strombolian activity at the N vent in Bocca Nuova (BN-1) ejected incandescent material almost as high as the W crater rim. No eruptive activity was observed at BN-2, though it produced explosions deep in the crater. A new high-temperature vent producing gas emissions was noted on 1 October in the same place a fumarole had been observed the previous week. Ash emissions from NSEC were sometimes accompanied by ejected incandescent material. Gas emissions increased at Voragine Crater from a vent that formed on 7 August 2016 on the E rim of the crater. NEC produced frequent brown-gray ash emissions, and ejected blocks and bombs, from a vent located in the W part of the crater floor.

INGV reported that during 3-9 September activity at Etna was characterized by gas emissions at the summit craters, with periodic Strombolian activity from vents in Bocca Nuova, Northeast Crater (NEC), and New Southeast Crater (NSEC). A few Strombolian explosions at NSEC were recorded on 5 September; an explosion at 0536 generated an ash plume that produced local ashfall around the vent and in the Valle del Bove, and quickly dispersed. A similar but less intense event occurred earlier that day, at 0316. Similar Strombolian events continued during 6-9 September, at intervals of a few hours. Strombolian activity at the N vent (BN-1) in Bocca Nuova occurred at 3-5-minute intervals, ejecting incandescent material that fell within the crater confines. Gas emissions were sometimes punctuated with ash emissions. Intense degassing was characteristic of the second vent (BN-2). Strombolian activity occurred at NEC, and a few times explosions were accompanied by ash emissions.

INGV reported that during 20-26 August activity at Etna was characterized by gas emissions at the summit craters, Strombolian activity, and ash emissions. Strombolian explosions continued from vents in Bocca Nuova (including from a vent that had opened on 16 August) and Northeast Crater (NEC). Activity at New Southeast Crater (NSEC) was characterized by modest and occasional explosions and Strombolian activity. Beginning at 1700 on 23 August Strombolian activity from the cone in the saddle between the Southeast Crater (SEC) and NSEC cone complex rapidly intensified; explosions ejected tephra 100-150 m above the vent rim. At 1730 Strombolian activity occurred at NSEC’s E vent, and a lava flow from the same vent traveled a few hundred meters towards the Valle del Bove. Just after 1730 lava overflowed the vent in the saddle cone and flowed N; Strombolian activity at that vent continued through the night and then stopped no later than 0620 on 24 August. At 0622 Strombolian activity from a vent on the S flank of NSEC produced a small lava flow that traveled a few dozen meters. During 25-26 August the activity at the saddle cone gradually decreased, and ash emissions were weak and occasional.

INGV reported that during 6-12 August activity at Etna was similar to the previous week, characterized by gas emissions at the summit craters, Strombolian activity, and ash emissions. Strombolian explosions continued from vents in Bocca Nuova, and were particularly visible at night. Activity at Northeast Crater (NEC) consisted of frequent ash emissions and Strombolian explosions. Explosions at the E crater on the E flank of the New Southeast Crater (NSEC) generated gray-brown ash plumes that sometimes rose several hundred meters above Etna’s summit and quickly dissipated.

Based on two field inspections and webcam data, INGV reported that during 30 July-5 August activity at Etna was characterized by gas emissions at the summit craters and Strombolian activity. Three vents at the bottom of the Bocca Nuova crater were active, with gas emissions rising from two vents and sporadic Strombolian activity occurring at a third. Three vents were also active at the bottom of Northeast Crater (NEC); one produced ash emissions, one steam emissions, and Strombolian explosions at the third ejected incandescent material as high as the crater rim. After several months of quiet conditions, explosive activity resumed at the E crater on the E flank of the New Southeast Crater (NSEC). The activity began at 0608 on 1 August with a brownish-gray ash emission that rose several hundred meters above the summit. The event was followed by more ash emissions and then Strombolian activity in the evening.

INGV reported that during 16-22 July gas plumes rose from Etna’s summit crater; low-energy Strombolian activity also occurred from three vents at the bottom of Northeast Crater (NEC) and from two vents in Bocca Nuova. The activity was mostly confined to the craters, with material falling back into the craters, though occasionally incandescent lava was ejected about 100 m above the crater rim. The activity sometimes generated ash emissions that dissipated near the summit.

INGV reported that during 9-15 July gas emissions continued to rise from Etna’s summit craters. Visibility of the fairly continuous, low-energy, Strombolian activity deep within the Northeast Crater (NEC) and Bocca Nuova was sometimes hindered by gas emissions. Ejected incandescent material fell back into the crater. Strong and prolonged roars (up to several tens of seconds) from NEC were sometimes accompanied by gray-brown and reddish ash emissions. Collapses of the crater’s inner walls widened the crater. Gas emissions from New Southeast Crater (NSEC) were weak and sometimes pulsating.

INGV reported that activity at Etna during 22-28 January was relatively unchanged compared to the previous weeks and was mainly characterized by variable-intensity gas emissions from the summit craters. Weak and sporadic ash emissions rose from the New Southeast Crater (NSEC). On 22 January ash fell in areas on the S flank and also in Catania, though the crater which produced the ash emission was unknown due to poor visibility.

INGV reported that, although weather conditions often prevented views of Etna during 27 November-3 December, gas emissions from the summit craters were visible along with ash emissions from a series of explosions from the vent in the saddle between the Southeast Crater (SEC) - New Southeast Crater (NSEC) cone complex.

INGV reported that at 1820 on 26 April lava began effusing from a vent on the S side of Etna's New Southeast Crater (NSEC), traveling to the base of the cone. Around 2220 Strombolian activity ejected material from the vent and the lava flow headed towards the Valle del Bove. A lava flow on the N side of the cone began at 0220 on 27 April and traveled NE towards the Valle del Leone. Ash emissions were visible at 0320. Strombolian activity began to diminish around 1230 and eventually ceased. The lava flows advanced until around 1600; phreato-magmatic explosions occurred in areas where the NE flow interacted with snow.

INGV reported that the effusive eruption from a vent in the saddle between Etna's Southeast Crater (SEC) - New Southeast Crater (NSEC) cone complex ended during 8-9 April. During 10-11 April lava effused from a vent located at the S base of the SEC. The lava flow traveled SE and flowed several hundred meters down the W wall of the Valle del Bove. In the evening of 13 April short-lived explosive activity occurred at a vent at the S flank of the SEC cone at about 3,200 m elevation. That vent and another at 3,150 m elevation produced lava flows for a few hours. Lava also effused from the vent at the S base of SEC, 3,010 m elevation, during 13-14 April, traveling SE and down the W wall of the Valle del Bove on top of the flows from 10-11 April.

INGV reported that the eruptive episode that began on 15 March from a vent in the saddle between Etna's Southeast Crater (SEC) - New Southeast Crater (NSEC) cone complex was characterized by Strombolian activity and ash emissions. Lava had traveled down into the Valle del Bove from the W rim; by the afternoon of 18 March almost continuous collapses of parts of the lava flow produced several avalanches of incandescent material which reached the base of the wall. Explosive activity had greatly declined earlier that morning. The collapsing stopped by early the next evening, 19 March, and the lava reached the base of the valley. That lava flow stopped advancing on 20 March though a few lava flows closer to the vent remained active the next few days.

On 21 March a new lava flow expanded SSW, but then stopped in the last days of March at an elevation of 2,300 m. A few more lava flows had followed a similar path in early April and then also stopped at the same elevation. Around 28 March a new pit crater had formed on the S part of the pyroclastic cone which had formed around the SEC-NSEC saddle vent. Strong glow form the pit crater was observed at night.

INGV reported that during the morning of 15 March lava began to flow down the S flank of Etna's Southeast Crater (SEC) - New Southeast Crater (NSEC) cone complex. Activity rapidly intensified at 0800, and by 1000 near-constant Strombolian explosions were generating ash plumes. The lava flow reached the base of the cone and traveled S. By late afternoon the lava was advancing on top of lava flows from the previous eruption. The intensity of the Strombolian activity reached a peak around 1840-1845, and by the evening both the eruptive activity and seismicity gradually diminished. Just before midnight a new lava flow began to effuse from a vent on the S flank of the cone. On 16 March at 1243 a phreato-magmatic explosion occurred at the front of a lava flow where it contacted an area covered with snow. An INGV-Osservatorio Etneo volcanologist was injured in the explosion, suffering minor bruises. A news article noted that about 10 people were injured during the event.

INGV reported that Strombolian activity at Etna’s New Southeast Crater (NSEC) cone had gradually intensified beginning at 1700 on 27 February. Lava overflowed the vent, quickly traveled down the S flank, and then slowed after reaching the gently sloping terrain. The flow spread SSW, towards the old cone of Monte Frumento Supino. During 27-28 February frequent Strombolian explosions ejected incandescent lava fragments up to 200 m above the vent. During a field survey on 28 February INGV-Osservatorio Etneo staff observed sporadic emissions of vapor and brownish ash from several vents within the NSEC. A new pyroclastic cone around the vent had grown higher than the highest points on the SEC (Southeast Crater) or NSEC. The lava flow slowly advanced on top of the snow, and by 1 March was about 1.5 km from the vent. By the late evening, activity declined, and the volcanic tremor amplitude rapidly diminished.

INGV reported that Strombolian activity at Etna’s New Southeast Crater (NSEC) cone that began on 23 January gradually began to intensify at 1700 on 27 February. A lava flow quickly reached the base of the cone and traveled SW towards Monte Frumento Supino. Ash plumes drifted E and caused ashfall in Zafferana and Linguaglossa.

INGV reported that on the morning of 15 December 2016 minor emissions of brownish ash rose from the vent in the saddle between the newer cone and the old cone of Etna's Southeast Crater (SEC). That evening some of the emissions were more energetic, ejecting incandescent material out of the crater which landed on the steep S flank of the SEC cone. Analysis of samples of the ejected rock revealed no new material, only older material from the conduit walls. During the following five weeks weak ash emissions from the vent were observed, without being accompanied by incandescence.

During the early morning of 20 January the saddle vent was again active, with small black ash puffs and thermal anomalies identified by the surveillance camera. People on the SEC observed ejected incandescent material along with ash and blocks. Cloudy weather prevented visual observations for a few days; the evening of 23 January was cloud free and mild Strombolian activity was observed, accompanied by frequent emissions of small, black ash puffs. The activity gradually intensified through the night, with some explosions launching incandescent material as far as the base of the SEC cone. Fluctuating incandescence was also visible at the 7 August 2016 vent of the Voragine (VOR) crater. The frequency of explosions and ash emissions increased on 24 January but then slightly decreased the next morning.

On 10 October INGV reported that during the past few weeks high-temperature degassing had continued from a vent that had opened on 7 August in the E portion of Etna's Voragine crater. In addition minor and infrequent ash emissions from old pulverized rock rose from a vent located on the upper E flank of the New Southeast Crater cone. In the early afternoon of 10 October an explosion occurred at the Bocca Nuova crater, in an area between the crater and the nearby Voragine crater. The explosion was recorded at 1526, and produced a distinct thermal anomaly and an ash puff that rose a few hundred meters. During the next few hours similar seismic events were detected although weather cloud cover prevented visual observations.

INGV reported that during mid-July weak ash emissions rose from a vent located high on the E flank of Etna’s New Southeast Crater (NSEC) cone. The emissions continued periodically until early August. Pulsating glow from mild, intra-crater explosions in the Voragine (VOR) crater was recorded during 7-9 August.

INGV reported that at around noon on 20 May Strombolian activity was visible at Etna's Voragine (VOR) crater and produced explosion noises audible across a large area on the S and E flanks. At night the activity intensified with explosions occurring at two or three vents, contemporaneous with renewed inflation of the summit area. Just after 0400 volcanic tremor amplitude rapidly increased and the Strombolian explosions turned into pulsating jets of lava, launching incandescent bombs 1 km S; an ash plume drifted SSE. A thermal camera also recorded activity from a vent in the S portion of Northeast Crater (NEC). Observers noted that a fracture had formed on the SE flank of the central cone. In addition, an effusive vent in the saddle between the cone and the old cone of the Southeast Crater produced a small lava flow that traveled towards the Valle del Leone. Activity decreased around 0500 and was over at about 0600.

On 22 May a vent on the upper E flank of New Southeast Crater cone produced a series of ash emissions which rose several hundred meters above the summit and dispersed. Some of the emissions had a thermal signature, indicating the presence of hot material. That evening Strombolian activity resumed at NEC; the rate and intensity of the activity fluctuated through the night. The strongest explosions ejected incandescent bombs up to a few hundred meters above the crater rim and onto the flanks. On 23 May sporadic ash emissions continued from the vent on the upper E flank of the New Southeast Crater cone.

INGV reported that intense Strombolian activity began at Etna's Northeast Crater (NEC) during the evening of 17 May. Incandescent bombs were ejected above the crater rim and tephra was deposited on the flanks. During the morning of 18 May this activity was accompanied by ash emissions that drifted E and rapidly dispersed. Weak incandescence from the 25 November 2015 vent, on the upper E flank of the New Southeast Crater (NSEC) was visible, and an inclinometer about 1 km NW of NEC recorded rapid inflation of the summit area.

Just after 1250 on 18 May cameras recorded the onset of activity at Voragine (VOR) crater, which within a few minutes evolved into a pulsating lava fountain. At the same time Strombolian activity at NEC diminished and dark ash emissions formed briefly. Ash plumes from VOR rose as high as 3.5 km above the crater and drifted ESE. During the afternoon lava overflowed from the W rim of the Voragine-Bocca Nuova depression, and traveled W within the summit area. A second lava flow, emitted from a vent located at the E base of the Northeast Crater (NEC), expanded into the N portion of the Valle del Bove. The second lava flow remained active until the early morning hours of 19 May. Later that morning, the volcanic tremor amplitude sharply increased, and contemporaneously loud and virtually continuous bangs were heard in populated areas to the E and S of the volcano. A dense eruption plume drifted E at an altitude slightly higher than 1 km above the summit of Etna. Ash and lapilli fell onto the E flank of the volcano, near an area affected by the tephra fall on the previous day. A few hours later images revealed a new lava flow from VOR traveling W. Eruptive activity continued at least through 0900, though the volcanic tremor amplitude had diminished.

INGV reported that during January eruptive activity at Etna was at low levels. During the last week of January and on 6 February ash emissions rose from a vent located high on the E flank of the New Southeast Crater (NSEC) cone. At 0422 on 23 February an explosion at Northeast Crater (NEC) ejected incandescent tephra several tens of meters above the crater rim, and produced a dark ash plume that drifted NE. A camera recorded lightning flashes in the plume. Weak ash emission rose from the crater during the rest of the morning.

INGV reported that after intense activity at Etna's Voragine Crater, Bocca Nuova, and the New Southeast Crater (NSEC) during the first 10 days of December, activity shifted to the Northeast Crater (NEC). During 9-10 December Strombolian activity was detected at NEC, with a few ejected incandescence bombs falling onto the outer flank and abundant ash emissions. Activity gradually diminished over a few days. On 13 December ash emissions rose from NSEC and on 18 December the Voragine Crater produced two brief ash emissions. Ash emissions began at 1100 on 28 December from a vent located high on the E flank of the NSEC cone. The emissions ceased in the afternoon; very minor and sporadic explosions continued from the same vent during the following days.

INGV reported that after a progressive intensification of activity during the evening of 2 December, an eruption at Etna's Voragine Crater peaked between 0330 and 0410 on 3 December. During the peak period sustained lava fountains rose over 1 km above the crater with some jets of hot material rising 3 km high. An ash plume rose several kilometers high and drifted NE, causing ashfall in Linguaglossa, Francavilla di Sicilia, Milazzo, Messina, and Reggio Calabria. Activity had almost ceased by dawn. This event was among the largest in the last 20 years, similar to large events occurring at the same crater on 22 July 1998 and 4 September 1999.

At about 1000 on 4 December renewed activity at Voragine Crater was characterized by tall lava fountains and an ash plume that rose 7-8 km high. The ash plume had a mushrooming top and produced deposits of coarse-grained pyroclastic material on the upper SW flank above 2 km elevation. Ashfall was reported in Giarre-Zafferana Etnea on the E flank. The activity was accompanied by frequent ash emissions from a new pit crater that had recently opened on the upper E flank of the New Southeast Crater (NSEC) cone. Two more events occurred at Voragine Crater during 4-5 December, between 2130 and 2215 on 4 December and 1555 and 1635 on 5 December, again producing tall lava fountains and many-kilometer-high ash plumes.

During the evening of 5 December activity at Voragine Crater progressively diminished. Between 0300 and 0400 on 6 December surveillance cameras recorded the onset of vigorous Strombolian activity from the vent on the E flank of the NSEC cone. Repeated collapses of both old and new material from the cone's flank generated hot avalanches that traveled a few hundred meters E towards the Valle del Bove. Ongoing effusive activity through the day produced two lava flows; one advanced NE for less than 1 km and the other advanced E. Strombolian activity continued through the evening. On 7 December a second vent on the E part of NSEC was also active, and the main lava flow had advanced 4 km. During the early morning hours of 8 December Strombolian activity progressively diminished and then ended. Later that afternoon weak Strombolian activity and ash emission were observed at the Northeast Crater.

INGV reported that the new episode at Etna’s New Southeast Crater (NSEC) that began on 12 May continued the next day. At 0410 on 13 May a series of small collapses accompanied the opening of three vents, along a fracture oriented E-W, below the E rim of NSEC, one of which effused a small lava flow. At 0800 a fracture at the vent propagated 200 m from the rim down the cone within 10 minutes. This event was accompanied by collapses, along with reddish ash ejection onto the summit area and the high S flank. Strombolian activity increased that night and was characterized by almost continuous Strombolian explosions, sometimes accompanied by ash emissions. This activity continued during 14-15 May. Ash plumes rose a few hundred meters and dispersed with the wind; minor ashfall was reported in areas from the S to the NE. A single lava flow traveled NE towards Mt. Rittman, and then E towards Mt. Simone where it formed two branches. One branch approached the base of the N wall of the Valle del Bove while the other traveled W to a distance 5 km from NSEC. Activity decreased on 15 May and ceased on 16 May.

INGV reported weak Strombolian activity inside Etna’s New Southeast Crater (NSEC) during 11-12 May, mainly from a depression on the E part of the crater and sometimes from a second vent in the middle of the crater. This activity was preceded by a sudden increase in tremor amplitude and intense degassing during 2-3 May; visible activity did not accompany the episode, and after a few days tremor returned to normal levels.

INGV reported that on 12 April two explosions from the W part of Etna’s Bocca Nuova Crater were recorded within a 3-minute period beginning at 1505. Resulting ash puffs rose a few hundred meters above the crater and dispersed.

INGV reported that a new eruptive episode at Etna began on 31 December and lasted through the morning of 2 February. Poor meteorological conditions prevented views of the summit area during the first 36 hours of the eruption. During improved viewing conditions on the evening of 1 February, volcanologists observed Strombolian activity from a single vent in the saddle between the cones of the Southeast Crater (SEC). Explosions occurred every few seconds and ejected incandescent bombs 200 m high. At the same time a vent at the base of the southern SEC cone issued a lava flow that traveled 2 km S, dividing into two branches. At dawn on 2 February the Strombolian activity produced a dense ash cloud that drifted E. At about 0550 emissions stopped and volcanic tremor suddenly decreased.

INGV reported that in the evening 14 January weak Strombolian activity was recorded at Etna's Voragine Crater and Northeast Crater. The next day occasionally pulsating ash emissions rose from Northeast Crater and drifted SE. Ash emissions continued through 17 January; cloud cover prevented observations of the summit area on 18 January.

INGV reported that at night during 6-7 January the frequency of Strombolian explosions at Etna's Voragine Crater decreased; however, some of the explosions ejected incandescent pyroclastic material outside of the crater and onto the W and SW flanks. On 7 January many of the small explosions generated brown ash plumes that rose a few hundred meters above Etna's summit and quickly dissipated. Strombolian activity increased on 8 January, possibly from two vents within the crater. Pyroclastic material continued to be ejected out of the crater. Early on 9 January Strombolian activity again decreased and gave way to ash emissions that rose several hundred meters. Ash emissions continued the next morning, decreased, and had almost completely ceased by late morning. Ash emissions resumed in the afternoon and were sometimes accompanied by Strombolian explosions. Ash emissions on 13 January were continuous in the morning but then decreased in frequency by the afternoon.

INGV reported that on 29 December, a day after a short but intense eruption, cameras viewing Etna recorded small ash emissions from New Southeast Crater (NSEC) and persistent glow from the saddle between the old and new SEC cones at dusk. During the night on 1 and 2 January cameras recorded intermittent flashes from Voragine Crater (one of four summit craters), indicating Strombolian activity there for the first time in nearly two years. At 0530 on 2 January explosions at NSEC generated ash plumes that drifted SW. On the evening of 3 January explosions ejected incandescent material 150 m above the crater rim.

INGV reported that starting at 1750 on 28 December Etna's New Southeast Crater (NSEC) produced a short but intense eruption characterized by lava fountains, lava flows, and an ash plume that drifted E and caused ash and lapilli fall in Milo, Fornazzo, South Alfio, and Giarre. Inclement weather prevented observations of the summit area so the erupting crater was not identifiable. Two lava flows traveled E and NE, towards the Valle del Bove. Tremor began to decrease at 1930, and indicated that the eruption was over at 2100.

INGV reported that from the afternoon of 7 October through 16 October Etna's New Southeast Crater (NSEC) produced weak and intermittent explosive activity; small ash puffs were rapidly dispersed by the wind. During some nights small Strombolian explosions ejected incandescent material a few tens of meters above the crater rim.

On 13 August INGV reported continued eruptive activity at New SE Crater of Etna, including Strombolian explosions accompanied by minor ash emissions. Lava continued to flow about 3 km NE towards Monte Simone. On 18 August INGV reported that the eruption at New SE Crater had ended on 15 August and lava flow activity had ceased as of 16 August.

On 9 August INGV reported that volcanic tremor decreased and an ash plume rose to 1 km above the July 25 area of Etna and returned to strong Strombolian activity in the evening. Strombolian activity increased at New Southeast Crater that was accompanied by small emissions of black ash that remained within the crater. The Etna Volcanic Observatory raised the Aviation Color Code to Red on 9 August as an ash plume rose to 4 km (13,000 ft) a.s.l. returning to Orange on 10 August. The Aviation Color Code was raised to Red on 11 August with strong Strombolian activity at Etna accompanied by significant ash.

INGV reported that a new, small fissure (tens of meters long) developed on the E flank of Etna during 5-6 July. The vent was located around 3,015-3,025 m elevation. Weak spattering from this vent fed a lava flow that extended ~100 m within the saddle of the NE and SE craters and cones. Weak and sporadic strombolian explosions and small ash emissions were observed during 6-7 July from New SE Crater, but by 11 July this activity had ceased.

Activity from the new fissure continued through 11 July with frequent strombolian explosions that were audible in nearby towns. The lava flow diverged, and the longest of the two branches extended ~1.5 km, reaching the bottom of Valle del Leone.

INGV reported that weak Strombolian activity at Etna's New Southeast Crater (NSEC) continued through 10 June. Some explosions ejected incandescent pyroclastic material a few tens of meters above the crater rim that occasionally fell onto the outer flanks.

On 14 June a new eruptive episode began within the NSEC, beginning with near-continuous Strombolian explosions and lava fountaining. Fine ash emissions were concurrent with lava that began to overflow the edge of the SE crater, forming a flow that continued downhill on the W wall of Valle del Bove. During the morning of 15 June the overflowing lava followed the fissure that had been formed on 28 November 2013. A spatter cone also formed in the E sector of the cone. During 14-15 June volcanic tremor increased sharply and remained at a medium-high level until 18 June when it returned to normal levels.

INGV noted that this activity was similar to the episode of effusive lava emissions observed during 14-16 and 19-31 December 2013 in terms of duration and intensity.

INGV reported that during the morning of 22 April sporadic and weak Strombolian activity resumed at Etna's New Southeast Crater (NSEC) and continued for the next few days. Some explosions ejected incandescent pyroclastic material out of the crater and onto the upper S and SE flanks of the cone. A few small collapses occurred on the cone's unstable E flank. The frequency and intensity of Strombolian explosions slightly increased late in the evening on 30 April. Degassing at the Northeast Crater also increased and thermal anomalies were detected by a camera.

Weak Strombolian activity continued to be detected through 1 May. During the night of 2-3 May incandescence was caused by weak high-temperature gas emissions and/or Strombolian explosions. The activity intensified on 4 May and some of the explosions ejected incandescent pyroclastic material onto the high S and SE parts of the NSEC cone.

INGV reported that during the night of 1-2 April emissions of minor lava flows from the NE base of Etna's New Southeast Crater (NSEC) cone decreased. Strombolian activity gradually intensified during the evening of 2 April and then decreased the next morning. Some collapses from the E flank of the cone were also observed that morning. Poor weather conditions prevented views of Etna for a few days, but by 7 April the lava flows had ceased and Strombolian activity had sharply declined. No activity was observed on 8 April.

INGV reported that Strombolian activity from Etna's New Southeast Crater (NSEC) cone ceased during the night of 26-27 March, after 64 days of persistent activity. Lava emissions from the lower side of the NSEC significantly decreased; on the evening of 28 March a small lava flow continued to advance but had stopped and was cooling the next day.

INGV reported on 24 March that during the previous week Strombolian activity with occasional diffuse ash emissions continued from one or two vents at the base of Etna's New Southeast Crater (NSEC) cone. Strombolian activity intensified during 18-22 March, producing more ash, and then decreased; no ash was emitted on 23 March. Lava flows originating from a vent on the upper wall traveled towards the upper part of the W wall of the Valle del Bove and also NE in the direction of Monte Simone.

INGV reported on 17 March that during the previous week Strombolian activity with occasional diffuse ash emissions continued from one or two vents at the base of Etna's New Southeast Crater (NSEC) cone. Lava flows originating from a vent on the upper wall traveled towards the upper part of the W wall of the Valle del Bove. During 14-15 March lava also flowed NE in the direction of Monte Simone.

INGV reported that during 6-10 March Strombolian activity and occasional diffuse ash emissions continued to rise from one or two vents at the base of Etna's New Southeast Crater (NSEC) cone. After several days of lava emissions from a vent on the lower part of the NSEC cone, during 5-6 March lava flows originated only from a higher vent and traveled 1.5 km towards the lower part of the W wall of the Valle del Bove. On 8 March sporadic emissions of hot material with small amounts of volcanic ash originated from Bocca Nuova.

INGV reported that during 28 February-4 March Strombolian activity and diffuse ash emissions continued at Etna's New Southeast Crater (NSEC). An unstable part of the lower E flank of the cone that collapsed on 11 February continued to produce small collapses with reddish ash clouds. Lava continued to flow from a vent on the lower part of the NSEC cone to the W wall of the Valle del Bove, and during 2-3 March the flows reached the base of the wall.

INGV reported that during 19-22 February Strombolian activity continued at Etna's New Southeast Crater (NSEC) and produced diffuse ash plumes. Lava continued to flow from a vent on the lower part of the NSEC cone to the W wall of the Valle del Bove. An unstable part of the lower E flank of the cone that collapsed on 11 February continued to produce small collapses with reddish ash clouds, and thermal anomalies.

INGV reported that Strombolian activity continued at Etna's New Southeast Crater (NSEC) and slightly intensified on 12 February. An unstable part of the lower E flank of the cone that collapsed on 11 February continued to produce small collapses and reddish ash clouds. Lava continued to flow from the cone towards the Valle del Bove, and by nightfall had reached the base of the steep W wall of the valley, then advanced on the flat land to the N of Mount Centenarians. Strombolian activity continued during 14-15 February. Lava emissions declined, but produced lava flows a few hundred meters long. At 1208 on 15 February an explosion generated a vapor-and-ash plume, and was then followed by more explosions from the same area. During the afternoon a small lava flow emerged from a new vent at the N base of the NSEC cone. The flow traveled 100 m towards the W wall of the Valle del Bove, and remained active the next day. During 16-17 February Strombolian activity continued to produce small quantities of ash. Lava continued to flow from the vent at the base of the cone.

INGV reported that during 4-5 February activity at Etna's New Southeast Crater (NSEC) was characterized by intermittent ash emissions accompanied by jets of incandescent pyroclastic material, and a constant emission of lava from one or two vents at the E base of the NSEC cone. The lava flows reached the base of the W slope of the Valle del Bove. On 6 February ash emissions ceased and small Strombolian explosions ejected incandescent pyroclastic material 100 m above the crater. On 7 February Strombolian explosions ejected material onto the flanks of the cone, and the next day ash puffs were observed. During 9-11 February activity continued to be characterized by Strombolian activity, periodic ash emissions, and advancing lava flows. At 0707 on 11 February a large quantity of reddish brown ash emitted from an area near the vents formed a very dense hot flow which quickly reached the base of the W wall of the Valle del Bove. Reddish brown ash emissions continued after the event.

INGV reported that on 28 January there was a gradual but steady decrease of activity at Etna's New Southeast Crater (NSEC). Weak and sporadic Strombolian explosions were detected at night, but then the thermal cameras recorded no anomalies associated with Strombolian activity. Lava flows from two vents at the base of the NSEC cone continued to effuse at a very low rate. Weather conditions almost entirely prevented observations of Etna from the morning of 30 January until the evening of 3 February. Late on 3 February observers noted that the lava flow remained active and was several hundred meters long. Almost continuous ash emissions from NSEC began at about 1300 on 4 February and continued into the night; about 5-10 ash puffs were separated by steam emissions. Ash plumes drifted E. After sunset jets of hot material were observed rising 100 m above the crater rim. At 2000 the ash emissions and injection of incandescent material ceased, but the lava flow continued and reached 1 km long.

INGV reported that during 4-13 January nearly continuous emissions of reddish ash from Etna's Northeast Crater were visible. Strong degassing continued at least through 22 January. Strombolian activity at New Southeast Crater (NSEC) began on the evening of 21 January, following 20 days of quiet. Some explosions generated very small ash emissions that barely rose above the crater rim. Late on 22 January a small lava flow from the vent on the high E flank of the NSEC cone traveled a few hundred meters in a few hours. Strombolian explosions ejected incandescent pyroclastic material onto the cone flanks. The frequency and intensity of the explosions decreased early on 23 January, and the lava flow stopped advancing. At 0105 a small puff of gas and/or ash from the E base of the cone heralded a new lava flow that traveled W towards the Valle del Bove. Weak Strombolian activity and the advancing lava flow continued during 24-28 January, although on 25 January the amount of ash produced by the Strombolian activity increased. On 26 January an ash plume drifted E. By evening the intensity of the Strombolian activity as well as the amount of ash in the emissions decreased. The lava flow was 4 km long.

INGV reported that during 31 December 2013-1 January 2014 lava flows from a vent located on the NE flank of the cone of Etna's New Southeast Crater (NSEC) continued to travel towards the N part of the Valle de Bove; the lava flows had been active since activity resumed on 29 December. On 3 January staff doing field work noted that the effusive activity had stopped.

INGV reported that on 28 December a helicopter overflight of Etna's New Southeast Crater (NSEC) revealed a crater floor partially covered with snow, and weak fumarolic activity on the N, W, and S crater rims. During the early morning hours on 29 December a camera recorded weak and sporadic incandescence from NSEC. Strong pulsating degassing also occurred at Northeast Crater. At 1115 NSEC produced a single Strombolian explosion, accompanied by an ash plume that rose 1 km and drifted E. After the explosion mild Strombolian activity continued and then progressively intensified in the evening. Frequent powerful explosions from two vents located within the crater were audible in a vast sector around the volcano. Diffuse ash plumes drifted NE. Contemporaneously, two lava flows are active, one from a vent on the E flank of the NSEC cone, and the second, fed directly from the crater, traveled down the NE flank of the cone.

INGV reported on the 20th paroxysm in 2013 from Etna’s New Southeast Crater (NSEC) began with a strong explosion at 0925 on 14 December from a vent near the NE rim which generated an ash plume that rose 2 km. Intermittent ash emission gradually turned into Strombolian activity. After 1413 the activity intensified; small ash puffs were produced and Strombolian explosions ejected incandescent pyroclastics onto the flanks of the cone. Over the next few hours activity continued to intensify and a diffuse ash plume drifted ESE. At 2323 lava overflowed the SE crater rim and flowed towards the Valle del Bove. During the night explosive activity continued to intensify, and by 0330 on 15 December Strombolian activity was intense and virtually continuous. After 0500 explosive activity started to diminish, and at 0550 the volcanic tremor amplitude and the intensity and frequency of Strombolian explosions showed a sharp drop. Explosive activity continued to produce copious amounts of ash until about 0830. According to a news article, the ash emissions caused the cancellation of more than 20 flights in and out of the Catania airport.

At 0610 and 0633 two vents opened within the deep trench cutting the SE flank of the NSEC cone (where lava was still flowing from the crater since the previous evening), the first just a few tens of meters below the crater rim, the second about 100 m further downslope. Both vents initially produced lava fountains for a few minutes, with jets a few tens of meters high, and then produced lava flows that descended through the trench.

Between 0945 on 15 December and the morning of 16 December ash emission varied in frequency and intensity, related to variations in the intensity of the Strombolian activity at NSEC. During the afternoon and evening of 15 December explosive activity lasted a few to a few tens of minutes and Strombolian activity became significantly more intense or passed into pulsating, low lava fountains. The more intense periods produced greater amounts of ash leading to ashfall in populated areas on the E and SE flank, from the Milo-Zafferana area toward the Ionian coast. At night during 15-16 December lava flows continued to flow towards the Valle del Bove. A shift in wind direction caused ashfall in areas SE, S, and SW.

On 15 December small and periodic ash emissions also occurred from Northeast Crater. Small thermal anomalies detected with a thermal camera were detected during 15-16 December.

Strombolian activity at the NSEC continued through 16 December at a slowly decreasing rate and with numerous minor intensifications that generated diluted ash plumes. Lava effusion also gradually diminished, but at about 1430 on 16 December, a short fissure opened on the lower NE flank of the NSEC cone, producing a small lava flow which advanced a few hundred meters. Explosive activity finally ceased around midnight on 17 December. Very slow lava effusion continued, at a gradually decreasing rate, from the fissure on the NE flank of the NSEC cone, through the night of 17-18 December.

INGV reported on the 18th paroxysm occurred from Etna’s New Southeast Crater (NSEC) late in the afternoon on 28 November. Weather clouds prevented observations of the event. Vigorous Strombolian activity on 2 December produced a plume comprised mostly of gas drifting N. The activity intensified during the evening and evolved into a paroxysmal event that finished by 2330. The event was characterized by tall lava fountains, lava flows, and a plume laden with pyroclastic material that drifted NNW and deposited ash and lapilli in that area. The lava flows traveled SSE, SE, NE, and S. The next day sporadic explosions continued, inclement weather however prevented visual observations.

INGV reported on the morning of 23 November the 17th paroxysm occurred from Etna’s New Southeast Crater (NSEC), five days after the previous one. The episode was characterized by a rapid evolution from Strombolian activity to lava fountains, an ash plume that rose several kilometers and drifted NE, and lava flows that were significantly less extensive than those emitted during the 16-17 November paroxysm.

INGV reported that weak Strombolian explosions from Etna’s New Southeast Crater (NSEC) were visible on 13 November. INGV-Osservatorio Etneo staff visited the area the next day and noted that explosions were heard one to three times per minute, and during times of good visibility no pyroclastic material was ejected. Sporadic ejections of incandescent pyroclastics were observed after nightfall. Early on 16 November Strombolian activity gradually intensified; however, only pulsating puffs of vapor, but no ash, were produced.

On 17 November a new paroxysmal eruptive episode was characterized by violent Strombolian activity and pulsating lava fountains, emission of lava flows that traveled S, ESE, and NE, and the formation of an eruption column charged with pyroclastic material that drifted NE. The episode ended with a long series of powerful explosions and loud bangs heard tens of kilometers away. Strombolian activity continued until the late evening; after nightfall, a small lava flow issued from an effusive vent located on the lower E flank of the NSEC cone.

INGV reported that ash emissions from Etna’s New Southeast Crater (NSEC) occurred at 1251 and 1254 on 6 November and were quickly dispersed by the wind. During 6-7 November several phases of frequent explosions produced ash puffs visible during the daytime and ejected incandescent material from Strombolian activity was visible at night. On 8 November the explosions occurred at intervals of several hours, producing small ash plumes that rose a few hundred meters above the summit and drifted ENE.

A culminating phase of lava fountains, ash emissions, and lava flows began at 0500 on 11 November, after about 10 hours of gradually intensifying Strombolian activity. Weather conditions prevented visual observations, but a strong increase in the volcanic tremor amplitude was detected. The phase of maximum intensity lasted about 7.5 hours, ending around 1130; the cessation of lava fountaining was followed by a long series of powerful explosions that generated loud bangs heard mostly in the N sector of the volcano. Ash and lapilli fell in areas E and NE. A voluminous lava flow expanded S from the NSEC, and two smaller lava flows traveled ESE and NE. Vigorous Strombolian activity continued, with explosions at intervals of 1-2 minutes, which launched incandescent material as high as 150 m above the crater rim. At night during 11-12 November Strombolian activity ceased and the lava flows were no longer active.

INGV reported on 5 November that, after 10 days of relative quiescence, the New Southeast Crater (NSEC) showed signs of activity. A brief glow seen on surveillance cameras was followed by ash emissions. The event was accompanied by a strong seismic signal detected by summit seismic stations. Poor weather prevented additional observations.

Sezione di Catania - Osservatorio Etneo reported that on 26 October Etna's New Southeast Crater (NSEC) produced a new episode of lava fountaining, six months after the previous paroxysm. A gas plume laden with pyroclastic material rose several kilometers above the summit and drifted SW, affecting population centers as far as the Caltanissetta area. According to a news article a representative from Catania airport noted that the eruption caused the closure of nearby airspace before dawn through the early morning.

Lava emitted from the saddle between the two cones of the Southeast Crater advanced S, destroying two wooden shacks at Torre del Filosofo. Another smaller lava flow descended the SE flank of the NSEC cone, partially filling the deep collapse scar formed during the 27 April 2013 paroxysm. At 1019 vigorous ash emissions from the Northeast Crater formed a dark brown plume that rose 1 km; ash emissions from that crater continued through late evening. Lava fountaining from NSEC continued through the late morning and was then followed by a long series of powerful explosions audible to many tens of kilometers away. Strombolian explosions occurred in the late evening. Lava flows continued to advance the next day.

Sezione di Catania - Osservatorio Etneo reported that a series of small and sporadic ash emissions from Etna's New Southeast Crater (NSEC) began during the morning of 3 September, marking the end of four months of complete quiescence. Weak Strombolian activity from NSEC was observed during the early morning of 6 September. At daybreak small ash puffs were emitted once or twice per hour. The same morning intense incandescence emanated from Bocca Nuova. The report stated that since early May only degassing from the summit craters was noted, along with usual bangs and rumblings from deep within the conduit of the Northeast Crater (NEC), which during the past few weeks had become more continuous and louder.

Sezione di Catania - Osservatorio Etneo reported that the thirteenth lava-fountaining episode of 2013 began at Etna's New Southeast Crater (NSEC) on 27 April. Activity increased on 21 April and was characterized by Strombolian explosions and frequent ash emissions. Eruptive activity and the volcanic tremor amplitude gradually increased in the evening of 26 April. Just after sunset on 27 April lava fountains rose 300-500 m, and lava flows from the SE and NE flanks of the NSEC cone and from the saddle between the two Southeast Crater (SEC) cones traveled S and N.

Sezione di Catania - Osservatorio Etneo reported that the thirteenth lava-fountaining episode of 2013 began at Etna's New Southeast Crater (NSEC) on 27 April. Activity increased on 21 April and was characterized by Strombolian explosions and frequent ash emissions. Eruptive activity and the volcanic tremor amplitude gradually increased in the evening of 26 April. Just after sunset on 27 April lava fountains rose 300-500 m, and lava flows from the SE and NE flanks of the NSEC cone and from the saddle between the two Southeast Crater (SEC) cones traveled S and N.

Sezione di Catania - Osservatorio Etneo reported that the eleventh lava-fountaining episode of 2013 began at Etna's New Southeast Crater (NSEC) on 18 April. Activity increased on 16 April with ejected incandescent tephra and small ash puffs from a vent inside NSEC, followed by weak Strombolian explosions. Strombolian explosions became more frequent and intense on the morning of 18 April and then were almost continuous by 1300. During the next two hours lava fountains developed and a dense plume drifted SSW. Ash and lapilli fell in between the villages of Ragalna, Belper, and Paterno, as well as the tourist area "Etna Sud." Lapilli-fall was a few centimeters deep and clasts were at most 5 cm in diameter. Three lava flows were produced; the largest flowed through the deep notch in the SE rim of the crater and traveled 4 km towards the Valle del Bove. The interaction of the lava with snow led to rapid melting, generating small lahars. The two other lava flows originated in the saddle between the two SEC cones; one traveled N and the other S. After the lava fountains ceased, strong explosions were heard the rest of the day. On 19 April explosions produced little puffs of ash and ejected hot tephra.

The twelfth episode occurred two days later during the late afternoon of 20 April. Intermittent explosions ejected incandescent tephra and generated small ash puffs on 19 April. During the evening a large dark plume rose from NSEC, and sporadic Strombolian explosions were observed. The explosive activity ceased in the late evening, but shortly afterwards the lower of the two effusive vents at the base of the NSEC cone produced a lava flow that traveled 1.5 km towards the Valle del Bove. Around 1700 ash puffs rose from the crater, followed by incandescent tephra ejected at 1713. Within a few minutes sustained lava fountains were observed, along with a tall eruption plume that drifted E. Ash and lapilli fell over a wide area to the E, including along the Ionian coastline, just S of Guardia Mangano, up to Fiumefreddo, including the towns of Taormina, Ripon, and Mascali, and further upstream, including Santa Venerina, Zafferana, Milo, and Sant'Alfio.

On 20 April several lava flows on the W wall of the Valle del Bove interacted with the snow, generating explosions and lahars. Around 1815 lava-fountain activity decreased and turned into explosions and ash emissions. At 1840 the paroxysm was over. In the evening, the lava flow emitted from the effusive vent at the base of the SE part of the NSEC cone was still well-fed. Poor weather conditions prevented visual observations until the evening of 21 April, when surveillance videos showed sporadic Strombolian explosions accompanied by small ash puffs at the NSEC, and the emission of a small lava flow from the base of the cone.

Sezione di Catania - Osservatorio Etneo reported that the tenth eruptive episode of 2013 began at Etna's New Southeast Crater (NSEC) on 8 April with sporadic ash emissions occasionally accompanied by incandescent material. A large explosion at 2252 was heard up to 15 km away. The ash emissions continued for about 48 hours. In the late afternoon on 10 April Strombolian activity began, producing minor ash emissions during some of the explosions. In the morning on 11 April Strombolian explosions occurred about every 2-5 seconds, ejecting incandescent pyroclastics several tens of meters above the crater rim. Strombolian activity increased slowly in intensity and frequency of explosions throughout the day; contemporaneously the volcanic tremor amplitude continued to show a gradual rise. Late in the afternoon frequent and very powerful Strombolian explosions occurred every 1-2 seconds and were widely audible around the volcano. Jets of incandescent pyroclastics often rose 200 m above the crater rim and generally contained minor amounts of ash. Around 1840, a small amount of lava flowed over the deep breach in the SE rim of the crater. In the late evening it stopped and showed evidence of cooling.

Around 0110 on 12 April another small lava flow traveled S then SE from the saddle between the two cones of the Southeast Crater (SEC). Throughout the night powerful explosions alternated with intermittent, low lava fountains. At daybreak on 12 April a dense eruption plume containing relatively minor amounts of pyroclastic material drifted ESE. Until about 1025 on 12 April, all eruptive activity occurred exclusively at one or two closely spaced vents within the NSEC, then lava was emitted from two vents at the NE base of the NSEC cone. At 1024 a flow of hot pyroclastic material from the same area traveled about 2 km NE in less than 1 minute.

In the meantime, eruptive activity continued at the NSEC with frequent, powerful Strombolian explosions and emission of modest quantities of volcanic ash, which was rapidly dispersed. Lava emission from the SSE rim of the NSEC, the saddle, and the NE base of the cone remained active. Shortly before 1200, the eruptive activity changed from low lava fountains to Strombolian explosions and intermittent ash emissions. Vigorous ash emissions resumed at 1214 both from the main vent of the NSEC and from the saddle vent. Expulsion of blocks, bombs and ash from the saddle vent continued until 1234, when the main vent of the NSEC reactivated, and for the next nearly 20 minutes both vents were the source intense ash emissions.

The activity shifted back entirely to NSEC, entering into the true paroxysmal phase of this episode at 1250 with sustained lava fountaining, accompanied by a return to high levels of the volcanic tremor amplitude. During the following 10 minutes, there was a considerable increase in the quantity of pyroclastic material in the plume, which drifted ESE. Tephra fall (ash and small lapilli) affected Fleri, Zafferana Etnea, Milo and S. Maria la Scala, although the quantity of fallout was much smaller compared to those of the previous paroxysms. Sustained lava fountaining continued for about one hour; a few minutes after 1400 the activity switched to sporadic Strombolian explosions and ash emissions, which gradually diminished in vigor. During 1400 and 1410, repeated collapses occurred on the SE flank of the NSEC cone, possibly from new vents at the base of the cone, from which a new lava flow traveled toward the Valle del Bove. The collapses generated avalanches and clouds ash. Explosive activity progressively diminished and completely ceased around 1700.

On the evening of 12 April, the lava flows emitted from the saddle and from the various vents on the flanks of the cone were incandescent and slowly moving; during the night, the surveillance cameras showed the cessation of all lava emission and the cooling of the flows. However, on the morning of 13 April, slow lava effusion resumed from the lower of the vents and a small flow advanced a few hundred meters. This flow ceased in the early morning hours the following day. Once more, on the evening of 14 April, there was a rather weak resumption of effusive activity from this vent, which ceased after a few hours.

Sezione di Catania - Osservatorio Etneo reported that on 3 April, after almost 18 days of relative quiet, the New Southeast Crater (NSEC) of Etna produced its ninth episode of lava fountaining. Activity had increased on 2 April when small grayish-brown puffs rose from NSEC. Cloud cover prevented further visual observations through the following night; however, sporadic glow suggested Strombolian activity. On the morning of 3 April, the volcanic tremor amplitude slowly increased and was accompanied by numerous explosion signals. Field observations revealed that at 1330 vigorous Strombolian activity was occurring at one or two NSEC vents, with jets of pyroclastic material rising up to a few tens of meters above the crater rim. The activity progressively intensified between 1400 and 1430, with frequent, powerful Strombolian explosions often generating loud bangs and launching great quantities of incandescent bombs (with diameters of many meters) onto the flanks of the cone. Shortly after 1435, ash emission started from the saddle vent (SV), followed a few minutes later by Strombolian explosions from the same vent. At 1450, a continuous jet of incandescent lava fountained up to 100 m, whereas the vents within NSEC continued to produce powerful loud explosions. At around 1505 a lava flow moved through the deep breach in the SE rim of NSEC and then traveled over the W rim of the Valle del Bove. During the same time interval, lava emissions started from SV, feeding a flow that went S.

Since 1430 the eruptive plume drifted SE and contained modest amounts of volcanic ash. At around 1540 ash emissions progressively increased and the volcanic tremor amplitude showed a rapid rise. Between 1540 and 1615 low lava fountaining continued from SV, whereas the vents within NSEC emitted intermittent, pulsating lava fountains. The incandescent jets from the vents within NSEC rose up to 400 m above the crater rim. At 1615, lava fountaining at SV intensified, with jets rising 400-500 m high. Explosions from the vents within NSEC continued, producing loud detonations every 1-2 seconds.

Lava fountaining significantly decreased between 1625 and 1628 when a new eruptive vent (NV) opened a few tens of meters to the W of SV, on the E slope of the old SEC cone, and emitted grayish-brown ash. A dense cloud of pyroclastic material emitted by NSEC vents and SV rose about 2 km high and drifted SE. Fallout of pyroclastic material affected almost the same area that had already been subjected to the heavy shower of lapilli on 16 March: Zafferana Etnea and Santa Venerina on the SE flank, and the N part of Acireale plus a number of smaller villages to the N at the S margin of Giarre, in the Ionian area. The deposit was thinner than that of 16 March, and the dimensions of the lapilli were notably smaller.

Between 1630 and 1640, the eruptive activity reached a new peak of intensity with sustained lava fountains from SV and powerful explosions from the vents within the NSEC. At 1637 a thermal surveillance camera recorded a pyroclastic flow from the NE flank of the NSEC cone. Two lava flows emerged from the same area and traveled toward the Valle del Bove.

Sezione di Catania - Osservatorio Etneo reported that Strombolian activity continued at Etna's Voragine Crater after the 5-6 March paroxysm at the New Southeast Crater (NSEC), but was rarely detected by 14 March. Sometime during the afternoon of 15 March, numerous explosion signals were detected, and the volcanic tremor amplitude increased through the evening. After nightfall, incandescence from the NSEC was visible, and nearby residents (on the SE flank) heard loud bangs coming from the crater. This activity continued during the night and through the following morning. At daybreak on 16 March, numerous gas rings formed by the more powerful explosions were recorded by the monitoring cameras and photographed by observers in the field.

During the early afternoon of 16 March, activity started to intensify more rapidly, and frequent jets of incandescent lava were launched up to150 m above the crater rim. At about 1800 lava started to flow through the deep breach in the SE rim of NSEC. Approximately 15 minutes later, explosions became progressively more energetic, ejecting incandescent bombs onto the outer flanks of the cone; contemporaneously, the quantity of pyroclastic material (ash and lapilli) in the eruptive plume increased. Ash plumes drifted SE.

Between 1830 and 1845, the eruptive activity changed from Strombolian explosions to lava fountaining, with the highest jets rising 600-800 m above the crater rim. The eruption column rose about 2 km above the summit of Etna before it was blown SE by the strong winds. Around 1900, several lightning flashes within the eruptive cloud were observed. Large, incandescent bombs and scoriae were deposited on the entire cone of the NSEC and the adjacent areas to the S and SE. Observations made after the end of the paroxysm revealed that a lava flow was emitted from the area of the saddle between the SEC cones. During the phase of most intense lava fountaining, numerous volcanic bombs fell onto the pyroclastic cones formed during the 2002-2003 eruption, up to 2 km from the NSEC. In this phase, the lava fountains were also spectacularly visible from the town of Randazzo, on the NNE flank, about 15 km from the NSEC.

Heavy tephra fall, mostly in the form of scoriaceous lapilli, affected the SE flank; on the W headwall of the Valle del Bove this material was still incandescent. Further downslope, in the towns of Zafferana Etnea, Santa Venerina, and a number of villages to the N of Acireale, the tephra fallout formed a continuous deposit of scoriaceous lapilli, which in the N portion of Zafferana Etnea locally was up to 10 cm thick. Many clasts in this area had diameters from 5 to 8 cm, and more rarely up to 10 cm. Numerous car windshields, skylights, and roof tiles were broken. Even on the Ionian coast, the deposit consisted largely of lapilli, with only a minor fraction of ash.

The activity started to decrease around 1904, and at 1910 the activity evolved into violent explosions that ejected broad fans of large, incandescent bombs, accompanied by loud bangs and detonations. These explosions ceased at 1920, but at 1927 two particularly powerful explosions ejected large incandescent rock fragments towards the SW at least 1.5 km from the crater. A few weaker Strombolian explosions occurred shortly after 1930. At 0449 on 17 March, a series of explosions started at Voragine, which continued for about 5 minutes, generating strong thermal anomalies that were recorded by the thermal surveillance camera, and probably produced small ash puffs. During the next few hours, weak glow coming from Voragine was recorded by a monitoring camera. There were also small collapses and slides of still hot material from unstable portions on the NSEC cone, which generated minor quantities of ash.

Sezione di Catania - Osservatorio Etneo reported that during 5-6 March a new episode of lava fountaining occurred at Etna's New Southeast Crater (NSEC). On 5 March an explosion from the vent in the W part of NSEC was detected at 1854 and ejected incandescent bombs several tens of meters above the crater rim. This explosion was followed by similar ones, initially separated by intervals of 15-20 minutes, but then gradually became more frequent. As the evening went on, the activity at NSEC progressively intensified; likewise, the volcanic tremor amplitude started to rise. Around 0012 on 6 March, lava started to flow from the saddle between the two SEC cones; a few minutes later, an eruptive fissure with several vents opened in the lower portion of the saddle. Lava jets became continuous around 0017, forming a fountain that rose 200-300 m above the crater rim. Various vents were active within the NSEC, in the saddle area, and at the base of the saddle, from where a voluminous lava flow expanded S and SE. Around 0100 this lava flow had reached the area of Belvedere station. Lava also flowed through the breach in the SE rim of NSEC.

For the next 30 minutes, lava fountaining continued with jets rising 600 to 800 m above the crater. An eruption column heavily laden with pyroclastic material rose several kilometers above the summit and drifted NE. Deposition was intense on the upper NE flank of Etna, covering the area of the Valle del Leone, Pizzi Deneri, and Serra delle Concazze with incandescent bombs and scoriae. Further downslope, at Linguaglossa, scoria with diameters of several centimeters fell, and ash and lapilli fall was also observed at Piedimonte Etneo, Fiumefreddo, Taormina, and other towns along the Messinian Ionian coast.

Just after 0100, and within a few minutes, explosive activity nearly ceased, with only a few weak Strombolian explosions. During the same time, however, an eruptive vent opened on the lower E flank of the NSEC cone, which produced vigorous spattering and a well-fed lava flow that advanced SE. This activity continued at a slowly decreasing rate for a few days and ceased altogether on 9 March.

At the Voragine, Strombolian activity continued after the 5-6 March paroxysm at NSEC, but then alternated with short episodes of intense Strombolian activity and nearly totally quiet intervals. These oscillations were reflected in the volcanic tremor amplitude, which during the 24 hours following the paroxysmal episode at the NSEC showed about twenty small peaks.

Sezione di Catania - Osservatorio Etneo reported that seismic activity at Etna's Bocca Nuova Crater (BN) gradually increased at night during 26-27 February. Copious, puffing emission of dense vapor from BN observed at sunrise slowly grew more energetic over the next few hours. Between 1030 and 1045, the volcanic tremor amplitude rapidly rose; at the same time an eruption column largely composed of vapor formed and hot material was ejected. The plume contained moderate amounts of reddish-brown volcanic ash, mostly during 1215-1220 and 1314-1316, which led to the fall of small quantities of very fine ash on the SE flank between Zafferana and Santa Venerina. These ash emissions likely resulted from collapse or sliding of unstable material on the steep inner crater wall; the eruptive vent, which lies in the SE part of the crater, is leaning against the wall and the rapid accumulation of pyroclastic deposits in that area might have facilitated collapses.

During the phase of most intense eruptive activity, INGV staff carrying out fieldwork in the summit area, noted that volcanic bombs fell outside the crater rim onto the SW flank of the central cone. Intense explosive activity was also observed within the Voragine (VOR), which since early October 1999 had not exhibited magmatic activity. The activity began to diminish around 1320, evident from a reduction in the volcanic tremor amplitude; at 1430, the episode was essentially over, even though strong degassing continued at both BN and VOR. During the evening of 27 February vigorous Strombolian activity in VOR was reported by observers on the W and E flanks. Minor and discontinuous Strombolian activity also occurred at BN which often launched incandescent volcanic bombs up to 150 m above the crater rim. The activity continued through the night, into at least early 28 February, when NSEC also started showing signs of renewed activity after more than four days of repose.

Weak explosive activity and sporadic weak ash emissions at New SE Crater (NSEC) were observed during the early morning hours of 28 February. Weak Strombolian activity in the W part of the NSEC appeared at 0917. Thirteen minutes later Strombolian activity was occurring at the main vent in the center of NSEC. Contemporaneously, the volcanic tremor amplitude started to rise. During the following hour, eruptive activity gradually increased, while the volcanic tremor amplitude increased rapidly; at 1030, at least three vents were erupting including the former "pittino" to the W, in the saddle between the old SEC cone and NSEC cone. This activity generated a dense gas plume containing modest quantities of ash. At 1040 lava flowed through the through the deep breach cutting the SE crater rim; 25 minutes later, explosive activity increased dramatically and lava fountains rose about 100 m above the crater rim. From 1115 on, frequent powerful explosions generated visible shock waves and heavily showered the NSEC cone with large volcanic bombs. The activity intensified further between 1117 and 1122, accompanied by a conspicuous cloud of vapor and brown ash issued from the saddle between the two SEC cones. This cloud marked the progressive collapse of a large portion of the saddle, which destroyed nearly all of the SW flank of the NSEC cone, and parts of the E flank of the old SEC cone. Voluminous amounts of lava flowed from the deep notch left by the collapse, expanding first S and then SE, in the direction of the Belvedere monitoring station. Lava was also emitted from the eruptive vents at the base of the NSEC cone; this lava mixed with the flow emitted directly from the NSEC toward SE.

Sustained lava fountaining, with intense pyroclastic fallout and the generation of a huge cloud of gas and ash, continued at maximum intensity for about 20 minutes. The ash cloud drifted E, leading to abundant ash and scoria fall in the Milo-Fornazzo and Giarre-Riposto areas. At 1142 the activity started to diminish, although dense clouds of vapor and grayish-brown ash were emitted from the collapsed saddle area. At about 1150, the activity at the "pittino" became phreatomagmatic; vapor and ash were emitted and hot, wet blocks that formed spectacular vapor trails were ejected. Shortly after 1200, explosive activity at the NSEC ceased, whereas lava emission continued from the collapsed saddle area as well as from the SE flank of the NSEC cone, at a slowly diminishing rate. The lava flows continued to advance during the night of 28 February-1 March, and ceased sometime during 1 March. Strombolian activity within the VOR continued without significant variations, and was observed by INGV staff during a field visit on the morning of 1 March, at least through 1500 on 3 March.

Sezione di Catania - Osservatorio Etneo reported two episodes of lava fountaining from Etna's New Southeast Crater (NSEC) on 20 February. The first episode's most energetic phase occurred during 0150-0235, producing an ash plume that drifted ESE, small lahars, lava flows from the fissure cutting the SE crater rim, and lava flows from a new eruptive fissure which opened on the lower SW flank of the cone. After the cessation of lava fountaining, weak spattering and low-rate lava emission continued from two small fissures at the SE base of the cone. During the late morning, eruptive activity at NSEC re-intensified. The second episode began just after 1415 and ended at about 1450. An ash cloud drifted E, and new lava flows traveled towards the Valle del Bove, following the paths of previous flows. Slow-moving lava flows from the fissures at the SE base of the NSEC continued into the next day, accompanied by sporadic, weak Strombolian explosions at the NSEC.

At 0233 on 21 February a new vent opened high on the W slope of the Valle del Bove and produced a lava flow that melted snow, creating lahars and voluminous steam plumes. At about 0300 Strombolian activity resumed at the NSEC; the activity became virtually continuous by 0440 and produced jets of incandescent lava that rose at most 100 m above the crater rim. Cloud cover periodically prevented observations, but at 0540 the clouds were illuminated, and at 0545 a lava flow emerged from the cloud cover. An ash plume drifted N, causing ashfall in the area between Randazzo (15 km NNE) and Linguaglossa (7 km NE), at Patti on the Tyrrhenian (N coast of Sicily), and as far as Lipari (more than 80 km N). Scoria clasts up to 15 cm in diameter also fell in Linguaglossa. After the cessation of this fourth paroxysm in just over two days, effusive activity continued from the eruptive vents, feeding two lava flows that traveled 2.5 km down to the base of the steep W slope of the Valle del Bove. This activity, accompanied by sporadic small Strombolian explosions at the NSEC, continued into the morning of 22 February.

At about 0700 on 22 February, rising tremor amplitude signaled the onset of a new episode, but from Bocca Nuova. Between 0730 and 0815 many vapor puffs and thermal anomalies were observed. Cloud cover prevented clear views, but mild Strombolian activity was likely occurring within the crater. At about 0815 the volcanic tremor amplitude started decreasing and the gas emissions from Bocca Nuova became less conspicuous. Contemporaneously, the emission of lava from the two effusive vents at the SE base of the NSEC cone and below the Belvedere station decreased and completely ceased during the afternoon of 22 February.

At about 1625 on 23 February thermal anomalies were detected from NSEC and 15 minutes later lava was visible from the vent at the SE base of the NSEC cone. By nightfall the lava flow and Strombolian activity were visible from populated areas on the S and E flanks, although cloud cover hampered views. Activity intensified during 1900-1930; lava fountains rose at most 150 m above the crater rim and a well-fed lava flow spilled through the breach in the SE crater rim and traveled toward the W slope of the Valle del Bove. Within the next 10 minutes jets of lava rose 500-800 m above the rim. During that time, the emission of pyroclastic material increased dramatically, forming a dense plume that drifted NE; the entire NE flank, from the NSEC to Pizzi Deneri and beyond, was covered with a sheet of incandescent material. Eyewitnesses also reported the fall of large clasts, some incandescent, in the area of Monte Baracca, ~5 km NE of the NSEC. High lava fountaining continued for about 35 minutes. At 2014 the height of the lava fountains rapidly decreased, and two minutes later, the activity had changed to Strombolian explosions that ejected incandescent tephra up to 100 m above the crater rim. At 2030 all explosive activity was essentially over. During the morning of 24 February, the lava flow emitted from NSEC continued to be fed, probably by one or more vents near Belvedere.

Sezione di Catania - Osservatorio Etneo reported that during the evenings of 13 and 14 February a camera recorded incandescence from Etna's New Southeast Crater (NSEC). In the early hours of 15 February incandescent bombs were ejected just higher than the crater rim. Strombolian activity gradually intensified on 17 February along with volcanic tremor amplitude. Small Strombolian explosions occurred every 1-2 seconds at daybreak, launching coarse-grained pyroclastic material a few tens of meters above the crater rim. After reaching a peak around 0700, activity started to diminish; a few hours later, the volcanic tremor amplitude returned to background levels, and by the evening there was no sign of eruptive activity.

Shortly before midnight on 18 February, the clouds dissipated from the summit area, revealing continuous weak Strombolian activity within NSEC. During 0000-0200 on 19 February the volcanic tremor amplitude rose gradually, then distinctly increased after 0200. Contemporaneously, the eruptive activity started to intensify from one vent in the center of the crater. About 10 minutes later, lava started to overflow through the deep notch in the SE crater rim, expanding slowly toward the steep W slope of the Valle del Bove. At 0457 the small pit crater that had formed on the SW rim of the NSEC on 27 August 2012 started to emit ash, and repeated rim collapses generated small landslides. Activity of the main vent within the crater rapidly increased, and at 0503 a lava fountain rose about 200 m above the summit of the cone.

During the interval from 0503 until 0507, several vents became active along a fracture from the pit crater to the notch in the SE crater rim. A dense cloud of ash rose and drifted E. Next, the entire NSEC cone was subjected to heavy fallout of coarse-grained pyroclastic material. The main lava flow advanced SE, and a small lava flow that developed on the flank below the pit crater traveled along the March 2012 fracture zone between the old and new SEC cones. At 0515 lava fountains rose 300-500 m above the crater rim and produced bombs and spatter that covered the S flank of the cone. Small avalanches of this incandescent material generated ash clouds. During 0516-0518 the S flank of the cone was veiled by a dense curtain of fallout from the lava fountains. At the same time, dense vapor clouds rose from the upper E flank of Etna, generated by the copious fallout of incandescent pyroclastic material onto the snow.

At 0519, a more substantial avalanche of fresh material detached from the S flank of the cone, generating a small pyroclastic flow that expanded a few hundred meters first S and then E. At 0536 a thermal monitoring camera recorded a lahar from near the Belvedere area, which was followed by a broad lava flow that descended the steep slope and reached the base after less than 20 minutes. During its descent, the lava continued to melt snow, producing numerous small lahars. At 0550 a second lava flow N of the first also generated lahars. Finally, at about 0600, a third lava flow, which generated a lahar, descended the W slope of the Valle del Bove to the S of the first flow.

Lava fountaining from the pit crater started to diminish around 0525, and at 0535 transitioned into ash emissions alternating with brief jets of incandescent lava. At 0545, one single vent, in the central portion of the NSEC, continued to produce lava fountains about 200 m high. A few minutes later, however, a new lava flow started to descend the lower SE flank of the cone, possibly after the opening of a new eruptive vent in the lower part of the notch cutting that sector of the cone. Surveillance cameras showed brief lava fountaining at that site, before all lava fountaining ceased shortly before 0600. During the interval between 0600 and 0615, the activity was characterized by emission of a dense ash plume with frequent jets of lava and powerful explosions, which launched large glowing bombs beyond the summit of the old Southeast Crater cone. After 0615 only ash emission persisted. At 0622 a puff of ash was emitted from the Bocca Nuova; shortly thereafter, ash emissions from the NSEC diminished notably and became discontinuous; the last, weak puffs of ash were observed around 0710. Slow lava emissions continued for a few more hours from the lowermost vent, which had opened shortly after 0547 on the SE flank of the cone. During the late afternoon of 19 February, small ash puffs were again emitted from the Bocca Nuova. Tephra fell in a narrow area extending from the NSEC towards the E, including Milo and Fornazzo (10 km E), Giarre (16 km E) and Riposto (18 km E).

Sezione di Catania - Osservatorio Etneo reported that during the days following the 2 February eruptive episode at Etna intermittent emissions of small quantities of ash were repeatedly observed from both the New Southeast Crater (NSEC) and Bocca Nuova Crater.

Volcanologists visited Etna on 5 February and observed the recent changes that had taken place at Bocca Nuova Crater, notably the growth of a pyroclastic cone surrounding the eruptive vent in the SE portion of the crater. This cone had grown at least 50-70 m, about halfway up the inner crater wall, and was leaning into the wall. The entire crater floor was covered with recent lava.

Later in the evening of 5 February a webcam recorded weak Strombolian activity at NSEC, which continued until daybreak the next day, and then became invisible due to deteriorating weather conditions. Seismic data showed an instantaneous increase of tremor at 1020; volcanic tremor amplitude rose to a peak within a few minutes after the start of the activity, and began to descend after less than one hour. People in the ski area on the NE flank of the volcano briefly saw a dense but ash-free gas plume rising from the summit.

After the episode on 6 February through most of 8 February sporadic ash emissions from NSEC were observed, although poor weather conditions often prevented observations. On the evening of 8 February weak glow from NSEC was intermittently visible. At 2100 a webcam recorded fluctuating glow from within Bocca Nuova which became more intense over the next 10 minutes. Around 2125 jets of incandescent lava were repeatedly seen rising above a thick blanket of clouds drifting over the summit area of Etna. Contemporaneously, the volcanic tremor amplitude rose sharply. After 2200 the volcanic tremor amplitude began to decrease, whereas the eruptive activity continued without showing signs of diminishing until about 30 minutes later. After 2230 on 8 February and during 9-10 cloud cover prevented observations. A brief ash emission was observed on 10 February.

Sezione di Catania - Osservatorio Etneo reported that intense Strombolian activity at Etna's Bocca Nuova Crater began on the evening of 30 January and was the fifth episode of activity during a three-week interval that began on 10 January. Weak glow from a vent on the SE part of the crater floor was first observed at 1807. The glow became stronger and was visible to nearby residents; simultaneously volcanic tremor amplitude rapidly increased, and shifted from below the New Southeast Crater (NSEC) toward Bocca Nuova. Between 1900 and 1915 the activity intensified, and from 1920 onward jets of incandescent volcanic bombs and scoria nearly continuously rose higher than the crater rim. Some tephra was ejected 150 m above the rim.

During 1930-2000, lava fountains rose 100 m above the rim. Shortly after 2000, the fountain leaned SW and produced heavy fallout of incandescent bombs and scoria on the outer SW flank of the central summit cone, down to its base. At 2016 the fountain rose vertically and pyroclastic fallout outside the crater diminished.

Around 2030 the lava fountain started to wane; the incandescent jets became discontinuous and only rarely rose more than 100 m above the crater rim, except for one jet, at 2100, which rose 150 m above the rim. In addition, the volcanic tremor amplitude rapidly decreased and returned to normal levels in the late evening. After 2200, the incandescent pyroclastic jets no longer rose above the crater rim, and the glow became progressively less brilliant. During the night, however, weak eruptive activity continued on the crater floor, evident from a dull glow emanating from the crater. During the early morning hours of 31 January, the glow gradually faded away, and the episode ended with a series of sporadic, small ash emissions, the last of which was seen around 0641.

During 31 January-1 February ash emissions at New Southeast Crater (NSEC) were nearly continuous for intervals lasting from a few minutes to more than one hour. On 1 February small discrete "puffs" of ash rose from Bocca Nuova. At both craters ash plumes rose no higher than 100 m above the crater rims.

At 0300 on 2 February a camera recorded weak glow from NSEC then after 0330 sporadic small explosions ejected incandescent pyroclastic material up to a few tens of meters above the crater rim. The strongest explosions (at 0345, 0400, 0409, and 0411) ejected glowing bombs onto the flanks of the NSEC cone. Two minutes after the last of the explosions, weak glow appeared at Bocca Nuova that only lasted a short time; during the following 30 minutes, however, intermittent glow was recorded at both craters. At 0450 jets of lava rose above the rim of Bocca Nuova; at 0500 Strombolian activity became continuous, producing jest that rose many tens of meters above the rim. Small Strombolian explosions resumed in NSCE at 0512. Just after 0515 activity at Bocca Nuova started to increase rapidly; contemporaneously, the volcanic tremor amplitude showed a sharp rise. Lava fountains rose 120-150 m above the rim. Activity at NSEC started to decrease at 0530 then ceased just before 0600. Activity at Bocca Nuova decreased markedly between 0620 and 0630; weak intracrater activity continued for a few more hours and then by 0900 the episode was over.

Sezione di Catania - Osservatorio Etneo reported that two episodes of Strombolian activity from Etna's Bocca Nuova Crater occurred during the evenings of 16 and 18 January. Both began with a sudden increase in volcanic tremor amplitude. Poor weather conditions prevented direct observations; the only visible evidence was a bright glow illuminating the clouds covering the summit. On 18 January some clasts were ejected onto the S outer slope of the central summit cone.

On the early morning of 20 January volcanic tremor amplitude again rose, and was much more pronounced at the EBEL station about 700 m from the New Southeast Crater (NSEC) than at the ECPN station much closer to Bocca Nuova. Contemporaneously, there were reports of glow illuminating the clouds over the summit of Etna. Seismic and infrasonic data analyses suggested that the activity occurred at NSEC and consisted of mild Strombolian explosions, which ceased a few hours later.

The next episode began at 22 January. Tremor rapidly rose at 1840 again producing a stronger signal at the EBEL station. Glow from Strombolian activity was first recorded by a camera at 1856; the activity then became more clearly visible and the Strombolian explosions became more frequent. Incandescent bombs were ejected as high as 100 m above the crater rim. The strongest explosions were followed by abundant fallout of coarse-grained tephra onto the flanks of the NSEC cone. Eruptive activity continued for nearly 12 hours with minor fluctuations. Noises produced by the explosions were audible to residents on the E flank. At about 0600 on 23 January the tremor amplitude decreased and the last explosion visible on camera footage was recorded at 0635. During the hour following, a few small, sporadic puffs of vapor mixed with volcanic ash rose from the crater and drifted E.

The two episodes of Strombolian activity at the New Southeast Crater during 20 and 22-23 January represented the first emission of new magmatic products after a quiet interval of nearly nine months. During the past few months, signs of a possible reactivation of the crater were observed, starting with a dull glow coming from within the crater on 22 November 2012 and a series of small vapor and ash emissions during 25-27 December 2012. A short-lived episode of intense glow occurred on the evening of 3 January 2013.

Sezione di Catania - Osservatorio Etneo reported that during 22 November-early December 2012 weak glow emanated from Etna's New Southeast Crater (NSEC) caused by the emission of high-temperature gas. The glow was most intense during 1-2 December, then rapidly diminished and reappeared on 24 December. During 25-27 December sporadic and weak ash emissions from NSEC were accompanied by increased gas emissions. On the evening of 3 January a strong glow was briefly observed.

Vigorous Strombolian activity at Bocca Nuova Crater began at night during 9-10 January, three months after the last episode. At 0000 on 10 January a rapid rise in tremor amplitude was detected. Ten minutes later a video camera recorded the first incandescent burst in the E part of the crater, which progressively became stronger and more frequent. At 0350 jets of incandescent fragments rose significantly higher than the crater rim. In daylight the phenomenon was no longer visible via the surveillance cameras; the volcanic tremor amplitude remained elevated but started to decrease around 1200. In the early morning of 15 January volcanic tremor amplitude rapidly decreased, marking the cessation of Strombolian activity in the Bocca Nuova Crater.

Sezione di Catania - Osservatorio Etneo reported that on the evening of 21 November, weak glow was observed coming from within Etna's New Southeast Crater, caused either by the emission of high-temperature gas and/or Strombolian activity. Incandescence was also visible during the following nights, but was weak and intermittent.

A seismic swarm, consisting of around seventy events, occurred below the NW flank on 22 November, with epicenters located in the area of Monte Maletto. During a field visit to the summit area on the morning of 23 November, scientists did not hear sounds typically associated with Strombolian activity. In addition, volcanic-tremor amplitude did not show any significant variations.

Sezione di Catania - Osservatorio Etneo reported that on the evening of 2 October weak Strombolian activity resumed within Etna's Bocca Nuova crater, from the same vent on the SE part of the crater floor that was active in July and August 2012. On 3 October the activity was accompanied by a small inter-crater lava flow on the S part of the floor. Over the next few days the activity slowly intensified and a small new cone developed over the old cone formed during July-August, which had subsequently nearly completely collapsed. Volcanic tremor amplitude rapidly increased on the evening of 6 October at the same time eruptive activity intensified. Lava flowed towards the W part of the crater floor and lava fountains pulsated. The intensity of both the eruptive activity and the volcanic tremor amplitude peaked just after midnight on 7 October, and strong glow from the crater was observed from nearby populated areas. Some of the lava jets rose well above the crater rim. Activity decreased at 0430, and the next day dropped to the lowest levels recorded for many weeks.

Sezione di Catania - Osservatorio Etneo reported that a new phase of activity at Etna's Bocca Nuova summit crater started on 2 July, was virtually continuous for three weeks until 24 July, then rapidly decreased. During the first 10 days explosions were separated by quiet intervals lasting several minutes, then later became more continuous. During periods of the strongest activity, some lava bombs were ejected beyond the crater rim, landing on a gently sloping area E of the crater. Lava was emitted during 4-24 July from vents on the flanks of the pyroclastic cone that had begun to grow around the explosive vent. Rarely, lava was emitted from the main explosive vent. The lava progressively covered the crater floor next to the pyroclastic cone, which had grown 30-40 m, before the activity ceased on 24 July. Frequent ash emissions observed the next day were mostly due to explosions, and possibly from minor collapses of the summit area of the new cone. Concurrent with the decrease in eruptive activity, volcanic tremor amplitude dropped to background levels.

Episodic activity occurred during 26 July-11 August. At daybreak on 26 July, strong incandescence marked the onset of the first episode of activity within Bocca Nuova, which had the same characteristics as the previous activity; frequent Strombolian explosions were accompanied by lava flow emissions from a vent located on the W flank of the cone. The next day, the lava had almost completely filled the central subsidence pit on the floor of Bocca Nuova, and the cone had grown in height. The activity then decreased, and essentially ceased by the evening. There were intermittent ash emissions on 28 July, and volcanic tremor amplitude decreased to background levels.

The second episode, during 29 July-1 August, was characterized by small Strombolian explosions and lava emission from a vent located on the S flank of the pyroclastic cone. Volcanic tremor amplitude showed strong oscillations, but never reached the same peak levels associated with the 26-27 July episode. Volcanic tremor amplitude dropped sharply on 1 August, marking the cessation of the activity; a few ash emissions were observed the next day.

The third episode began on 3 August with a rapid increase in the volcanic tremor amplitude and strong incandescence from Bocca Nuova, signaling the beginning of Strombolian activity and lava emissions onto the crater floor. Activity ceased the next day, and was followed by weak residual activity within Bocca Nuova, which generated small ash clouds. The fourth episode began on 6 August and ended the next day, and showed essentially the same characteristics as the previous episode.

The fifth episode began on 10 August and was again marked by a rapid increase in volcanic tremor amplitude; at nighl, bright glow illuminated the gas plume rising from the crater, which was visible up to tens of kilometers away. Some of the explosions ejected incandescent pyroclastic material well above the crater rim. The activity started to decrease just after midnight and then ended on 11 August. Subsequently, ash emissions resumed, which were particularly frequent on 13 August, and which produced small, brownish-gray clouds of fine-grained tephra. Some of the ash emissions later that evening were accompanied by ejected incandescent material; volcanic tremor amplitude, however, remained at background levels.

Sezione di Catania - Osservatorio Etneo reported that on the evening of 2 July weak incandescence began to illuminate a gas plume rising from Bocca Nuova, marking the resumption of eruptive activity in this crater nearly one year after a short period of Strombolian and effusive activity. Observatory staff visited the crater the next day and observed Strombolian activity from a single vent, a few meters in diameter, at the base of the SE crater wall. Incandescent bombs and scoria ejected by the explosions fell back around the vent.

During the following days, the activity within Bocca Nuova continued with minor fluctuations; on 4 July, a few bombs fell outside the E crater rim, and on 8 July a small lava flow was observed on the flank of the pyroclastic cone growing around the active vent. Similar activity was also observed on 11 and 13 July. On 16 July, the amplitude of the volcanic tremor recorded by the summit seismic network showed a consistent increase, which was accompanied by an intensification of the activity within Bocca Nuova. During the night of 16-17 July, continuous, bright glow from the crater was visible from populated areas around the volcano.

Sezione di Catania - Osservatorio Etneo reported that the twenty-fifth paroxysmal eruptive episode since January 2011 took place at New SE Crater (New SEC) of Etna during the morning of 24 April following about 11 days of quiescence. During 21-22 April a series of small ash emissions observed by web cameras drifted downslope to the E. On 22 April prolonged emissions of white vapor rose from an area in the upper portion of the fissure that cuts the SE flank of the cone. At night several incandescent spots were visible on the NE and S crater rims, where hot gas heated the surrounding rocks. On 23 April a small thermal anomaly in the area of New SE Crater was produced by a tiny lava flow, which issued from the same vent that had produced the prolonged vapor emissions on the previous day. The lava flow slowly advanced a few hundred meters toward the Valle del Bove, but stagnated on the W rim. Another lava flow was produced later that day.

At around 1800 the effusive vent started to vigorously spatter. A second vent became active a few tens of meters further upslope, which initially ejected spatter, but activity rapidly evolved into frequent Strombolian explosions accompanied by a slow rise in the volcanic tremor amplitude. During the following hours, sporadic explosions were also observed from a vent located within the New SEC; the activity remained more or less consistent, while the volcanic tremor amplitude fluctuated at only slightly elevated levels. At 0210 0n 24 April the Strombolian activity showed a marked increase and turned into sustained lava fountaining at 0230. A plume with a heavy load of ash and lapilli rose a few kilometers, drifted NE, and produced ashfall 15-17 km NE in Linguaglossa, Piedimonte, and Presa. The upper part of the plume drifted E and produced pea-sized lapilli fall in the area between Fornazzo (10 km E) and Giarre (17 km E). The phase of most intense lava fountaining lasted approximately 25 minutes, from 0240 until 0305 on 24 April, after which the intensity rapidly decreased and returned to Strombolian activity, before ceasing completely around 0340.

Sezione di Catania - Osservatorio Etneo reported that the twenty-fifth paroxysmal eruptive episode since January 2011 took place at New SE Crater (New SEC) of Etna during the morning of 24 April following about 11 days of quiescence. The episode was characterized by lava fountains, emissions of ash and lapilli, and lava flows that descended the Valle del Bove.

Sezione di Catania - Osservatorio Etneo reported that the twenty-fourth paroxysmal eruptive episode since January 2011 took place at New SE Crater (New SEC) of Etna during the morning of 12 April following about 10 days of quiescence. The episode was characterized by lava fountains, emissions of ash and lapilli, and lava flows that descended the Valle del Bove and explosively interacted with snow cover.

Sezione di Catania - Osservatorio Etneo reported that the twenty-third paroxysmal eruptive episode since January 2011 took place at New SE Crater (New SEC) of Etna during the morning of 1 April following two weeks of quiescence. The episode was characterized by tall lava fountains from vents within the crater and on the SE flank of its cone, a gas-and-tephra plume, and lava flows descending toward the Valle del Bove. The paroxysmal phase lasted about 1.5 hours and ended just before daybreak. Ash and lapilli fell over the SE sector of the volcano, affecting mainly the area between Monterosso and Zafferana Etnea, but also the area between Acireale and Giarre, further downslope.

Sezione di Catania - Osservatorio Etneo reported that the twenty-second paroxysmal eruptive episode since January 2011 took place at New SE Crater (New SEC) of Etna during the morning of 18 March following two weeks of quiescence. Roaring from high-pressure degassing was heard on 16 March. The next day there was incandescence and multiple vapor clouds with minor ash content that rose from New SE Crater. In the early hours of 18 March the incandescence intensified due to Strombolian activity on the crater floor, and volcanic tremor amplitude rapidly increased. Strombolian activity continued to intensify, and just before 0700 lava flowed through the deep breach in the SE crater rim. At about 0825 the ash content in the gas plume rising from the crater became more significant and pulsating lava fountains from a vent on the crater floor rose about 100 m high. Shortly before 0900 two vents were active within the crater and a jet of lava was emitted from another vent within the breach in the SE crater rim.

During 0900-0915 lava fountaining was essentially continuous from all three vents. An intense shower of coarse-grained pyroclastic material falling onto the N and NE flanks of the cone generated avalanches and clouds of rock and dust, which traveled to the base of the cone. A plume rose 4-5 km above Etna and drifted E. Ash and lapilli fell mainly in the area between the villages of Zafferana Etnea and Sant'Alfio, extending toward the Ionian Sea between Riposto and Pozzillo.

The main lava flow descended the steep W slope of the Valle del Bove. Several lava lobes, however, took a more northerly path to areas covered with thick snow. The interaction of the lava and snow led to rapid melting of the snow, generating small lahars, and strong explosions that produced ground-hugging vapor-and-ash clouds resembling pyroclastic flows, which repeatedly descended on the floor of the Valle del Bove. The vapor-and-ash clouds rose 1-1.5 km above the floor of the Valle del Bove. This phenomenon continued intermittently for some time after the cessation of the lava fountaining and ash emission, until about 1130.

Lava fountaining and strong ash emission continued without significant variations until about 1040; afterwards the activity rapidly diminished in intensity, and the last ash clouds were observed around 1110. Similar to the previous episodes, the lava that flowed through the breach in the SE crater rim advanced for several hours after the cessation of the paroxysmal activity into the upper part of the Valle del Bove. The lava reached a distance of about 4 km from the source, stagnating S of Monte Centenari. A small lava flow, emitted from a fracture on the N flank of the cone, followed the same path as a flow emitted from the same fracture during the 4 March paroxysm, and traveled a few hundred meters.

Sezione di Catania - Osservatorio Etneo reported that the third episode of lava fountaining from Etna's New Southeast Crater (New SEC) occurred on the morning of 4 March and was more explosive than the preceding episode. The beginning of the eruption was characterized by a rapid increase in volcanic tremor amplitude coincident with Strombolian explosions that increased in intensity and frequency. Just after 0800 lava overflowed the deep breach in the SE rim of the crater and reached the SE base of the cone within 15 minutes, then advanced towards the W rim of the Valle del Bove. Explosive activity changed to continuous lava fountaining and an eruption plume developed at about 0830. Large pyroclasts fell on the steep flanks of the cone, causing avalanches.

At about 0850 small pyroclastic flows generated by the partial collapse of the eruption column mainly descended the NE flank, and somewhat down the S flank. A lava flow was emitted from a new eruptive vent on the upper SW flank of the New SEC cone and descended into the saddle between the old and new SEC cones. The lava interacted with snow, causing powerful explosions and small pyroclastic flows. These phreatic explosions generated jets of vapor and launched rock fragments to distances of several tens of meters. A lahar developed which traveled toward the "Belvedere" monitoring station, on the W rim of the Valle del Bove, and passed a few tens of meters to the N of the monitoring instruments.

A lava flow also issued from an eruptive fissure on the upper N flank of the cone and descended a few hundred meters to the NE, surrounding the N base of the cone. After descending the steep W slope of the Valle del Bove, the flow split into several branches on the more gently sloping terrain. These branches exceeded in length those of 9 February, reaching a total distance of about 3.5 km from the crater. Shortly after 1000, the activity started to diminish; lava fountaining ceased at 1032, two hours after the onset of the paroxysmal phase. The lava flow emitted from the fissure on the SW flank of the cone continued advancing for a few hours after the cessation of the activity.

The eruption column rose several kilometers above the summit of Etna. Ash and lapilli were carried NE by the wind, affecting the areas around Piedimonte, Etneo, and Taormina. Fine ash fell as far as the Messina area and southern Calabria. Again, the pyroclastic cone of the New SEC had grown in height, mainly on its N rim.

Sezione di Catania - Osservatorio Etneo reported that on 8 February the New SE Crater (New SEC) of Etna entered another paroxysmal eruption episode after about 12 days of Strombolian activity. During the afternoon of 8 February, the Strombolian activity increased in both frequency and intensity of explosions from the vent on the W portion of the crater floor, and occasionally from a vent to the E. After about 1900, lava began to spill into the deep breach on the SE crater rim and then descended to the base of the cone. The lava flow expanded around 2100 and the Strombolian activity slowly increased, turning into a discontinuous lava fountain around 2330.

On 9 February the pulsating lava fountain rose 100-500 m above the crater before a continuous jet rising 300-400 m above the crater. A cloud with a small amount of tephra rose 6 km above the summit and then drifted W, producing some pyroclastic fallout that was on the upper portion of the volcano. Around 0200 and 0400, lava fountains from the two vents within the crater rose as high as 500 m above the crater. The fountain from the E vent caused abundant fallout on the crater rim and E flank. A third vent, located in the breach cutting the SE crater rim, produced sporadic violent explosions that ejected bombs many meters in diameter all over the E portion of the cone.

The lava flow reached the W rim around 0130, descended the flank, and branched into three different flows that reached a distance of 3 km from the New SEC. Lava fountaining started to diminish around 0530, and then around 0545 sporadic jets rose as high as 300 m above the crater. At the same time, the vent in the SE crater breach produced strong explosions that again ejected bombs many meters in diameter. These explosions generated loud bangs that were heard all over Etna and small ash emissions that were reported on the W portion of Bocca Nuova. Around 0900 activity started to diminish and ceased abruptly at 1000.

Sezione di Catania - Osservatorio Etneo reported that an explosion at the New Southeast Crater (New SEC) of Etna generated an ash plume at that rose 400 m above the crater at 1906 on 27 January. An explosive ash emission at 1124 from the vent on the W portion of the crater floor was followed by others that were less intense. At 2140 an explosion ejected incandescent material in a narrow vertical jet that rose a few tens of meters above the crater. A small Strombolian explosion occurred at 2231. On 28 January sporadic ash emissions continued from New SEC.

Sezione di Catania - Osservatorio Etneo reported that on the night of 4 January the New SE Crater (New SEC) of Etna entered another paroxysmal eruption episode after about 50 days of quiescence. Several hours of Strombolian activity were observed on the SE and NE slopes starting at 2230 on 4 January. This activity continued into the early morning of 5 January when (around 0200) a small lava flow spread out into several branches at the SE base of the cone. Strombolian activity increased around 0400, generating lava fountains, from several vents within the crater that rose 100-150 m above the crater.

Significant ashfall and pyroclastic material fell onto the flanks at 0450 on 5 January. Around 0515 lava fountains generated a continuous eruption of ash-and-gas plumes that rose to an altitude of 2.1-2.4 km (7,000-8,000 ft) a.s.l. Small pyroclastic flows went a few hundred meters and lahars traveled down the NE, E, and S flanks of the cone. Around 0600 active vents along the N edge of the New SEC produced intermittent lava fountains. The most intense phase of the eruption occurred around 0620 when a strong explosion opened a vent on the top side of the SE cone, removed a portion of the SE crater rim, and generated ash plumes. The N slope of New SEC showed gravitational movement due to the amount of deposited pyroclastic material. Around 0630 activity started to diminish and ceased abruptly at 0730, but was followed by passive ash emissions that lasted until 0830.

Sezione di Catania - Osservatorio Etneo reported that the eighteenth paroxysmal eruptive episode of 2011 took place at the New SE Crater (New SEC) of Etna during the morning of 15 November. Thermal monitoring cameras at the observatory in Catania and at Montagnola, about 3.5 km S of the summit craters, recorded a small thermal anomaly at the lower end of the eruptive fissure on the SE flank of the cone at about 0700. The anomaly slowly grew in size and temperature, caused by the emission and expansion of a small lava flow. Mild Strombolian activity commenced at 0900 from within the New SEC, and spattering began from several vents along the fissure on the SE flank of the cone. This activity continued for nearly three hours, while increasing very slowly, and the lava flow spread out into several branches at the SE base of the cone, advancing only a few hundred meters. At about 1155, the activity markedly and rapidly increased both within the crater and along the fissure, and just after 1200 lava fountains and ash emissions rose from the crater. Lava fountains then rose from vents along the SE flank fissure. Bombs and scoria fell into the cone.

At 1230 ash emissions significantly increased, especially from a vent located in the SE portion of the New SEC, and a plume of ash and gas rose several kilometers above the summit and drifted SE. The most intense phase of the eruption occurred between 1245 and 1315 when jets heavily laden with incandescent bombs rose as high as 800 m above the crater. Pyroclastic material fell onto the New SEC cone, areas well beyond the base, and the nearby old SEC cone. During this phase explosions occurred from a vent on the N flank of the New SEC cone, likely the same vent that emitted small lava flows on 28 September and 8 October. At about 1325 the activity started to diminish and ceased abruptly at 1329, but was followed by passive ash emissions that lasted until just after 1400. Weak and discontinuous spattering accompanied by slow lava effusion continued for a few hours from a single vent in the central portion of the eruptive fissure on the SE flank of the New SEC cone.

Lava flows from the eruption traveled less than 4 km toward the floor of the Valle del Bove, immediately to the N of the Serra Giannicola ridge, stagnating to the SW of Monte Centenari. The New SEC grew in height by 10 m on the S side, bringing the total height of the cone to about 180-200 m above its base. Ash and lapilli deposits affected the SE flank, including the towns of Zafferana Etnea (10 km SE) and Acireale (20 km SE).

Sezione di Catania - Osservatorio Etneo reported that the seventeenth paroxysmal eruptive episode of 2011 took place at the New SE Crater (New SEC) of Etna during the evening of 23 October. Weak explosive activity was recorded at 1913, and at about 1935 small anomalies appeared in images recorded by thermal cameras. Explosive activity rapidly intensified at 1940 and by 2007 that crater was completely filled with lava. The lava overflowed through a breach in the E crater rim and traveled towards the Valle del Bove.

At 2026, Strombolian activity transitioned to continuous lava fountains that rose a few tens of meters above the crater rim. At 2036, a vent opened on the SE flank of the cone, producing a second lava fountain, leading to a significant increase in the lava effusion rate. The height of the lava fountains significantly increased after 2100, reaching 300 m above the crater. At about 2130, a third vent became active within the New SEC, possibly near the N rim. Abundant amounts of tephra fell on the E flank of the cone, forming a dense curtain, while large incandescent blocks rolled to the base of the cone on more gently sloping terrain. At approximately 2229 two lightning flashes near the crater were observed. After 2230 both effusive and explosive activity showed a marked reduction, changing again into Strombolian activity around 2300, and ceasing altogether at 2315.

The lava flow continued to advance towards the Valle del Bove until about 0040 on 24 October and stagnated just upslope of Monte Centenari (at 1,900 m a.s.l.). The area most heavily affected by the tephra (ash and small scoriaceous lapilli) fall was the E flank of Etna, including the N portion of the towns of Zafferana (about 10 km to the E), Milo (about 11 km ESE), and Fornazzo (10 km E), downslope across Santa Venerina (SE flank) and Dagala del Re (14 km ESE) to Giarre (~17 km E) and Riposto (18 km E), as well as nearby villages.

Sezione di Catania - Osservatorio Etneo reported that the sixteenth paroxysmal eruptive episode of 2011 took place at the New SE Crater (New SEC) of Etna during the afternoon of 8 October. A rapid increase in volcanic tremor amplitude was detected that morning, and at about 1300 weak and discontinuous Strombolian explosions were recorded by surveillance cameras. Two hours later, lava flowed from a deep notch in the SE rim of the crater.

At around 1545 vigorous Strombolian activity was observed from numerous vents along a short fissure on the SE flank of the cone, which had first been active during the 29 August paroxysm (the twelfth). Weather conditions deteriorated at about 1615, preventing direct observations of the Strombolian activity changing into sustained lava fountaining and ash emissions. However, this change was audible around 1630, and a dense ash-and-vapor plume rapidly rose above the weather clouds and drifted E. At the same time lava flows descended on the W slope of the Valle del Bove. Eruptive vents opened on the NE flank of the cone, approximately along the fracture that first opened during 8 September, and two small lava flows were emitted. The more voluminous lava flow traveled a few hundred meters downslope. The paroxysmal phase lasted a little longer than 20 minutes and ended around 1650. Ash emissions continued until 1945, when the volcanic tremor amplitude returned to levels similar to those preceding the paroxysmal episode, and the lava flow fronts appeared to stagnate. The ash cloud drifted ENE and produced ash- and lapilli-fall in a narrow sector from the Ripe della Naca area and the village of Puntalazzo (13 km E) to the town of Mascali (18 km E).

Sezione di Catania - Osservatorio Etneo reported that the fifteenth paroxysmal eruptive episode of 2011 took place at the New SE Crater of Etna during the evening of 28 September. That morning, noises originated from the New SE Crater. At 1600 volcanic tremor amplitude started to increase, and rhythmic ash emissions that rose from a vent located within the crater were occasionally accompanied by small Strombolian explosions. Starting at 1930 Strombolian activity progressively increased both in the intensity and frequency of explosions, and eventually became continuous. Bombs and scoria were ejected well beyond the crater rim. A small amount of lava flowed through the notch in the SE flank of the volcano at 2115 and was soon followed by explosions from a vent within the same area. The Strombolian activity waned for a few minutes then rapidly increased, forming a sustained lava fountain that rose as high as 800 m. At 2133 and 2134 two powerful explosions originating from a vent on the E rim of the crater created shock waves visible in the clouds above the crater and ejected large bombs hundreds of meters away. Lava fountains 100-150 m high commenced from a vent on the N base of the New SE Crater cone at 2136. The vent emitted a small lava flow soon after. At 2155 activity from all vents decreased, and between 2205 and 2210 all explosive activity ceased. Lava continued to flow until 2330.

The lava flow emitted from the SE flank of the cone reached the lower portion of the W slope of the Valle del Bove, somewhat SW of Monte Centenari. Ash plumes drifted SW and light ashfall occurred on the S flank of Etna, including in the towns of Nicolosi (14 km S) and Catania (27 km S).

Sezione di Catania - Osservatorio Etneo reported that the fourteenth eruption from Etna's New SE Crater in 2011 occurred during poor weather conditions on 19 September. Lava fountains rose from multiple vents within the crater and lava flows descended the W slope of the Valle del Bove. An ash plume drifted NE; ash and lapilli fell in an area to the N of Giarre (16 km E). The main phase of the event lasted from about 1020 until just after 1300. During the evening the lava flows were active and weak spattering continued from a vent at the SE base of the SE Crater cone.

Sezione di Catania - Osservatorio Etneo reported that the thirteenth paroxysmal eruptive episode of 2011 took place at the New SE Crater of Etna on the morning of 8 September. Prior to the episode, a few emissions of ash from the New SE Crater occurred on 6 September. Sporadic, very weak Strombolian explosions from the crater started during the late evening on 7 September, and then continued in a subdued manner through the night.

On 8 September a series of ash emissions were followed by a rapid increase both in the intensity and frequency of Strombolian explosions. Loud detonations were audible across a vast sector of Etna's densely populated SE to E flanks. Simultaneously the volcanic tremor amplitude sharply increased and shifted from below the NE Crater toward the SE Crater. The Strombolian activity turned into a pulsating lava fountain, accompanied by increasing amounts of volcanic ash. Lava fountaining and ash emissions became more vigorous. Lava flowed through a deep breach in the E crater rim and along the fracture that had opened on the SE side of the cone during 29 August. The lava overflow was accompanied by repeated collapse and rockfalls from unstable portions of the cone in that area. Later brief periods of repeated emissions of brown ash mixed with white water vapor occurred from two or three vents on the N flank of the New SE Crater cone, in an area of the lava overflows from the N rim of the crater that had started shortly after the onset of the activity. The paroxysmal activity ceased in the evening and was followed by a series of progressively more passive ash emissions. Lava flows descended on the W slope of the Valle del Bove; expansion of the most advanced lava fronts continued for some time after feeding of the lava had ceased, mostly due to gravitational flow. Small active lava flows were observed for many hours after the cessation of the paroxysmal activity, remaining confined to the immediate vicinity of the crater.

The pyroclastic cone that grew around the New SE Crater during the recent series of eruptive episodes had undergone significant morphological changes. The S and N crater rims had further increased in height, whereas degradation and mass wasting on the SE flank had become more conspicuous. A large chunk of rock on the lower SE flank was rotated and uplifted, forming a steep-sided "spine" about 20-30 m tall, with locally vertical and sub-vertical flanks.

Sezione di Catania - Osservatorio Etneo reported that nine days after the previous episode, the New SE Crater produced its twelfth paroxysmal eruptive episode of 2011 during the early morning of 29 August. The event was preceded by an explosion at 2252 on 27 August, and a series of ash emissions from the New SE Crater almost 15 hours later. Weak Strombolian activity visible during the evening on 28 August intensified during the night.

At 0115 on 29 August lava overflowed the rim through a breach in the E crater rim and traveled towards the Valle del Bove. Strombolian activity progressively intensified and two vents within the crater emitted pulsating lava fountains up to 100 m high. The lava fountains again increased in height and a dense plume rose a few kilometers above the summit before drifting toward the SSE. At the same time, a small lava flow issued from the area affected by the collapse of a portion of the cone's ESE flank during the 20 August paroxysm. At 0220 the SE flank of the cone fractured and exposed a line of new eruptive vents down to the base of the cone that produced lava fountains. A broad lava flow fed by the vents descended into the Valle del Bove, somewhat to the S of the lava flow emitted earlier during the episode. The lava fountaining from the vents within the crater turned into ash emission just after 0220, whereas the lowest vent on the new eruptive fracture continued to produce incandescent jets until 0250. Ash emission continued at a diminishing rate until about 0315.

Sezione di Catania - Osservatorio Etneo reported that eight days after the preceding episode, Etna's New SE Crater was the site of another paroxysmal eruptive episode on the morning of 20 August, the eleventh event of this type since the beginning of 2011. The event began on 18 August with increased gas emissions from New SE Crater. On 19 August a powerful explosion ejected incandescent bombs and produced a small ash plume. The event was followed by a few more minor explosions, all accompanied by an increase in volcanic tremor amplitude and focal shift from the NE Crater toward the New SE Crater. Throughout the day small dilute ash was emitted. During the evening weak Strombolian activity commenced, with small explosions occurring about every 30 minutes.

On 20 August weak but continuous incandescence due to lava emissions appeared in the crater. Strombolian activity intensified, and lava overflowed the rim through a breach in the E crater rim traveling towards the Valle del Bove. Almost five hours later lava fountaining generated heavy fallout of large pyroclastics onto the flanks of the cone. Dense plumes of gas and tephra rose 5-6 km from the crater and drifted SW, causing ash- and lapilli-fall in areas such as Paternò (22 km SSW), Ragalna (13 km SSW), and Biancavilla (16 km SW). Closer to the crater, in the Torre del Filosofo area to the S, clasts up to several tens of centimeters in diameter landed on the ground.

Light brown dust clouds appeared in an area on the lower E flank of the cone, where a small depression had formed a few hours after the 12 August event. Shortly thereafter, the continuous, intense ejection of pyroclastics onto the flanks of the cone generated avalanches resembling pyroclastic flows, which descended a few hundred meters beyond the base of the cone, mainly towards the S. In the meantime, the lower portion of the E flank of the cone began to slide and collapse under the push of lava from within the channel. A new lava flow issued from the collapsed area, taking a more southerly path than the lava emitted until then, and divided into numerous branches. Lava fountaining slowed later in the evening and eventually ceased, followed by ash emissions from the crater for a few minutes. A series of ash explosions lasted for five minutes in the early morning on 21 August. The morphological changes affecting the pyroclastic cone surrounding the New SE Crater were significant. Besides the collapse on the lower E flank of the cone, the S and NE rims of the cone had grown in height.

Sezione di Catania - Osservatorio Etneo reported that on 11 August sporadic ash emissions from Etna's New SE Crater produced small grayish-brown ash plumes. Thermal surveillance cameras revealed hot material in late-afternoon emissions. In the evening and throughout the night small Strombolian explosions were observed at intervals of a few tens of minutes. Early on 12 August, the day of the tenth paroxysmal eruptive episode of 2011, the Strombolian activity intensified and was accompanied by an increase in volcanic tremor amplitude. Strombolian explosions then produced dark ash clouds, and lava overflowed the E rim of the crater through a deep breach formed during previous eruptions. During the following 30 minutes or so Strombolian activity rapidly intensified, and formed a pulsating lava fountain about 100 m tall. Fifteen minutes later a dense column of ash rose above the lava fountain while large bombs and blocks fell onto the cone surrounding the New SE Crater.

During the most intense period three vents in the crater were active, two in the central portion and one close to the E-rim breach. Soon after, the two vents in the center of the crater emitted only ash, while the E vent continued to eject jets of incandescent lava. The activity completely ceased more than a half an hour later. The lava produced during the eruption descended the W slope of the Valle del Bove in numerous lobes; the most advanced lava fronts reached the base of the steep slope above Monte Centenari. Ash- and lapilli-fall affected a relatively narrow area between Zafferana (10 km SE), and the coastal area between Giarre and Acireale, on the SE flank.

Sezione di Catania - Osservatorio Etneo reported that the ninth paroxysmal eruptive episode from Etna in 2011 took place at the New SE Crater (located on the E flank of the old SE Crater cone and previously called the "pit crater") during the night of 5-6 August. Weak Strombolian explosions occurred in the crater during the afternoon on 5 August and gradually increased over the next few hours. At 2215 lava flowed over the E rim of the crater and towards the W slope of the Valle del Bove. Strombolian activity rapidly increased and formed a lava fountain that rose 100 m above the crater rim. Activity again intensified and jets of lava rose several hundred meters high. An eruption plume laden with ash and lapilli rose a few kilometers above the crater and drifted SE. At the climax of the eruption lava fountains exceeded 500 m in height. Just after midnight the incandescent jets diminished in height, continued to pulsate for about an hour, then further diminished. By 0215 on 6 August the eruption was substantially over. Ash-and-lapilli fall were observed in the SE sector of the volcano, between Zafferana (10 km SE) and Viagrande (16 km SSE) and between Acitrezza (23 km SE) and Pozzillo (19 km SE) along the Ionian coast.

Sezione di Catania - Osservatorio Etneo reported that in the evening of 28 July Strombolian explosions from the active vent on the E flank of Etna's SE Crater cone were weak and sporadic, then ceased during the night. Throughout 29 July the crater was quiet. In the early morning of 30 July intermittent incandescence from the crater gradually intensified and became more frequent, then was followed by intense Strombolian activity accompanied by loud detonations. Lava bombs ejected several tens of meters fell back into the crater or around the rim. A diffuse ash plume drifted E. A small lava flow on the E flank descended about 100 m then rapidly chilled. The activity was accompanied by a distinct increase in the mean amplitude of volcanic tremor that, along with the activity, abruptly decreased in the early afternoon.

Later that day the mean amplitude of volcanic tremor increased again along with Strombolian activity. A diffuse gas-and-ash plume again drifted E. Strombolian activity intensified and incandescent jets became continuous. At the same time lava flowed E and the effusion rate rapidly increased; lava flowed 3 km down the W slope of the Valle del Bove. The ash plume became more dense and ashfall was reported in the Ionian area (18 km E). During the most intense period, fragments of fluid lava were ejected 450-500 m above the crater and fell onto the flanks of the pyroclastic cone to distances of 200-300 m. Lava fountains jetted from at least two vents located within the crater and on the upper E flank, roughly aligned WNW and ESE. The activity ceased just after midnight. The event on 30 July was the eighth paroxysmal event in 2011.

Sezione di Catania - Osservatorio Etneo reported that the seventh eruption of Etna in 2011 occurred from the active crater on the E flank of the SE Crater cone. On 24 July vigorous Strombolian activity started within the crater, gradually increased through the night, and culminated on 25 July. Strombolian activity gradually turned into a pulsating lava fountain, accompanied by increasingly voluminous ash emissions. The fountain fluctuated between 250 and 300 m above the crater with a few jets rising 350 m. Lava flowed through a breach on the E crater rim and divided into multiple parallel flows that reached the base of the steep W slope of the Valle del Bove near Monte Centenari. Plumes drifted E causing ashfall between the villages of Fornazzo and Milo on the flank (10 km E), and the Ionian coast near Riposto (18 km E). The final phase of the eruption was characterized by a series of violent explosions that produced loud detonations heard in the E and SE sectors of the volcano.

Sezione di Catania - Osservatorio Etneo reported that on the evening of 11 July Strombolian activity began from Etna's Bocca Nuova crater, representing the first magmatic eruption since 2002. Incandescence was first recorded with a monitoring camera on the SE flank and was later visible from population centers in the SE sector of Etna. During the night of 12-13 July incandescence was more continuous and intense than during the previous night. At varying intervals, incandescent bombs that were ejected above the crater rim fell back into the crater. On 13 July volcanologists visited Bocca Nuova and observed a single large vent on the crater floor that was the source of the Strombolian activity. The strongest explosions ejected incandescent bombs several tens of meters above the crater rim. Most bombs fell back into the crater but some went over the rocky partition that divided Bocca Nuova from Voragine, and fell into the S portion of the latter.

In the evening of 15 July, volcanologists again visited Bocca Nuova and noted that the Strombolian activity had decreased slightly but within less than two hours had increased to levels greater than those observed on 13 July. Immediately to the W of the explosive vent, a lava flow was issuing from underneath a sheet of pyroclastic material deposited by the nearby Strombolian activity. The flow cascaded into a deeper depression in the W central portion of the crater floor.

On 16 July, a series of ash emissions from the pit crater located on the E flank of the SE Crater cone marked the resumption of explosive activity within the crater, and produced loud booming sounds that were widely heard in populated areas on Etna's flanks. On the evening on 18 July Strombolian activity increased and culminated into a new paroxysmal eruptive episode on 19 July. Lava flows traveled down the steep W slope of the Valle del Bove, following the same path as the lavas emitted during the preceding eruptive episodes, and stagnated at the base near Monte Centenari. Lava fountains rose 200-250 m and produced heavy fallout of fluid spatter, forming several lava flows. The largest lava flow descended the S flank of the cone reaching the base. A dense plume of gas and ash drifted E.

Sezione di Catania - Osservatorio Etneo reported that explosions from the pit crater located on the E flank of Etna's SE Crater cone were heard on the morning of 4 July. Incandescence was observed later that evening. Small Strombolian eruptions were recorded by a camera during 5-6 July.

On 7 July Strombolian activity gradually increased along with volcanic tremor amplitude. Small pyroclastic cones began to grow on the crater floor. During the next morning, volcanic tremor amplitude clearly increased. Shortly thereafter it abruptly decreased and the Strombolian activity completely ceased. During the morning of 9 July, Strombolian activity resumed and volcanic tremor amplitude rose rapidly. Around noon lava overflowed the E rim of the crater and followed the path of lava flows from the previous eruption, into the upper W part of the Valle del Bove. Later that day Strombolian explosions turned into a continuous lava fountain. A dense eruptive plume rose several kilometers high and drifted S and SE, causing ash and lapilli fall in populated areas including Trecastagni, Viagrande, and Acireale towards the SE, and between Nicolosi and Catania towards the S, forcing the closure of the Fontanarossa international airport in Catania. A few hours later volcanic tremor amplitude dropped to very low levels and all eruptive activity ceased.

Sezione di Catania - Osservatorio Etneo reported that on 13 June mountain guides heard loud hissing sounds coming from the interior of Etna's Bocca Nuova crater. After nearly six months of quiescence, ash emissions rose from Bocca Nuova the next morning. The emissions were composed of small ash clouds that seemingly originated from the central part of the crater and rose about 200-250 m above the crater rim, then drifted E. Thermal monitoring cameras showed no signs of incandescence in the emissions, which were observed for a few hours before meteorological clouds moved in and prevented further observations. During limited periods of good visibility from 15 to 17 June, sporadic small reddish-to-grayish-brown ash plumes were observed to occur every 5-15 minutes. Usual rhythmic emissions of gas and vapor from the Northeast Crater continued.

Sezione di Catania - Osservatorio Etneo reported that on 8 May mild and discontinuous Strombolian activity resumed at the pit crater located on the E flank of Etna's SE Crater cone. On 11 May volcanic tremor amplitude progressively increased, and at about 2030 lava started to overflow the E crater rim. The activity intensified during the following hours and, shortly before 0400 on 12 May, culminated with lava fountaining which generated an ash cloud that drifted SSE. The lava fountaining lasted around 2 hours, showing a gradual diminution around 0545; an abrupt drop in the volcanic tremor amplitude at 0610 marked the end of the eruptive activity.

Sezione di Catania - Osservatorio Etneo reported that on 8 May mild and discontinuous Strombolian activity resumed at the pit crater located on the E flank of Etna's SE Crater cone. Loud detonations were audible many kilometers away including in Monti Sartorius (NE flank) and in Zafferana Etnea (SW flank). After sunset, Strombolian explosions observed at intervals of 3-10 minutes ejected incandescent bombs up to a few tens of meters above the crater rim. During the night, some explosions ejected bombs well beyond the crater rim, down to the base of the cone that has grown around the crater during the recent events. Strombolian explosions continued without significant variations the next day.

INGV-CT reported that from 29 March through the first few days of April, a series of gas-and-ash emissions, rarely with minor incandescent material, rose from the pit crater on the E flank of Etna's SE Crater cone. On 8 April small Strombolian explosions occurred from two vents located in the W portion of the crater floor; ejecta were confined to the crater depression. On 9 April seismicity from the Strombolian activity increased throughout the day. In the afternoon, explosive activity commenced from two vents before lava flows covered the crater floor. Later that evening incandescent blocks appeared within a breach in the E rim of the crater, followed by a small overflow of lava. The lava flow advanced from the base of the SE Crater cone toward the W headwall of the Valle del Bove, as far as 1 km. At the same time small but frequent Strombolian explosions continued within the crater.

During the night of 9-10 April, the Strombolian activity within the pit crater gradually increased, as well as volcanic tremor amplitude. The lava flow continued to advance. On 10 April, activity and tremor amplitude significantly increased and culminated with vigorous lava fountaining. An ash-and-gas plume drifted SE, causing ashfall in areas downwind. The lava-flow emission rate also increased dramatically. A second lava flow covered the first and traveled down into the Valle del Bove, essentially following the same path as the lava flows of 12-13 January and 18 February. The lava flow encountered thick snow cover, leading to violent explosive interactions that generated pyroclastic flows, and resulted in spectacular vapor-and-ash plumes. The eruption declined rapidly after about 1500; no activity was observed later than the afternoon.

INGV-CT reported that on 16 February an explosive sequence, presumably from Etna's NE Crater, was detected on 16 February during the late evening. Cloud cover made direct observations difficult. On 18 February the thermal monitoring camera at Montagnola (EMOT) recorded anomalies from the pit crater located on the lower E flank of SE Crater cone. At the same time, the visible-light camera at Montagnola (EMOV) showed intermittent incandescence indicating Strombolian activity, and the seismic network recorded a rapid increase in the volcanic tremor amplitude. This eruptive episode lasted about 11 hours and produced pulsating lava fountains. Lava flows traveled E, following the same path as that of the 12-13 January event, in the direction of the Valle del Bove. Light ashfall occurred on the SW flank of the volcano.

On the evening of 12 January Strombolian activity from a pit crater located on the lower E flank of Etna's Southeast Crater cone intensified and volcanic tremor amplitude increased. Just after 2100 lava breached the crater's E rim and formed a lava flow that traveled towards the W wall of the Valle del Bove. During the night of 12-13 January, the pit crater produced lava fountains, voluminous lava flows that descended to about 1,630 m elevation (about 4.2 km from the vent), and an ash plume that rose several kilometers. The ash plume drifted S and caused ashfall on Etna's S flank and in population centers such as Nicolosi. Scoria several centimeters in diameter fell in Rifugio Sapienza, at 1,910 m elevation. The lava fountains were sustained initially and rose 300-500 m high, then pulsated and became less vigorous, and eventually formed one single jet that rose less than 100 m. Fountaining ceased at 0055 on 13 January. Emissions of ash during 13 January were generated in part by collapses within the crater and also by sporadic explosions within the conduit. On 14 January small landslides within the pit crater produced grayish-brown plumes.

After a few hours of quiescence on the evening of 3 January, very weak emissions of incandescent material (probably mostly hot gas with little or no solid material) resumed from the pit crater located on the lower E flank of Etna's Southeast Crater cone. Later that evening intense incandescence from the pit crater was reported by observers in the village of S. Alfio on the E flank of Etna. The emissions continued on 4 January at a rate of 4-6 events per hour, producing small plumes that appeared as thermal anomalies in thermal video footage. Activity ceased in the afternoon. Very weak emissions of incandescent material (probably again mostly hot gas) were observed on 5 January at the same rate as the previous day. A camera recorded white vapor plumes occasionally accompanied by some grayish-brown ash. Weak, intermittent incandescence from the pit crater was observed at night during 5-6 January. Emissions on 6 January occurred at a frequency of 3-4 events per hour, producing small plumes seen in thermal camera footage. That same day, intense degassing occurred from the W vent of Bocca Nuova (BN-1), and from the Northeast Crater, where pulsating emissions produced mushroom-shaped vapor plumes.

Starting in the late evening of 11 January, the seismic network recorded a slight increase in volcanic tremor amplitude. The amplitude peaked early on 12 January, and the source of the tremor shifted from a location N of the Northeast Crater towards the Southeast Crater. Weak Strombolian activity from the pit crater accompanied these changes on 11 January, and gradually became more intense on 12 January.

INGV-CT reported that during the first few days of December gas emissions from a large pit crater on the lower E flank of Etna's Southeast Crater cone nearly ceased. On 22 December at 0446 a strong explosion occurred at the W vent of the Bocca Nuova (BN-1). This event generated an ash plume a few meters high, which then drifted NE, causing light ashfall in areas as far as the town of Linguaglossa (17 km NE). On 23 December bluish gas rose from a vent at the base of the W wall of the pit, at the base of the Southeast Crater cone. Bright incandescence was intermittently visible on video footage. Inclement weather prevented clear observations that day and during the next few days. On 29 December extremely small amounts of incandescent material emitted from the pit crater were observed using visible and thermal cameras. The brief emissions (2-6 second intervals) were jets of mainly hot gas that barely rose above the rim of the pit crater. Inclement weather again prevented observations of the crater during 30-31 December.

During the late afternoon on 2 January, strong incandescence at the pit crater evolved into vigorous Strombolian activity. Frequent Strombolian explosions (1-3 per minute) ejected coarse-grained incandescent material a few tens of meters above the rim of the pit. On a few occasions, incandescent bombs fell outside the pit's rim, mainly to the S and E. The activity continued into the early morning then decreased markedly. Negligible quantities of volcanic ash were produced.

INGV-CT reported that a strong explosion from the W vent of Etna's Bocca Nuova crater at 0446 on 22 December produced an ash plume that rose a few hundred meters above the summit and drifted NE. The explosion of hot, though not incandescent, material and subsequent ash emission was observed and recorded by thermal cameras at Montagnola and Nicolosi (14 km S). Light ashfall was reported as far away as Linguaglossa (18 km NE). Small emissions of reddish-brown ash continued to occur before noon.

Following several days of ash emissions from Etna's Northeast Crater, INGV-CT reported that on 14 November there was a significant increase in both the frequency and volume of ash emissions. The emissions were intermittent, alternating with periods of gas-and-steam. Ash plumes rose a few hundred meters high and drifted first SW on 14 November, then NE during 14-15 November, and finally E on 15 November.

INGV-CT staff visited the summit craters on 15 November and saw a few millimeters of brown ash on the ground mainly to the S of Northeast Crater. Ash deposits were 1 cm thick on the rim of the crater. Ash emissions were accompanied by nearly continuous deep rumblings. The vent on the crater floor was at least 75 m in diameter compared to about 25 m in October.

INGV-CT reported that on 25 August a powerful explosion from Etna's Bocca Nuova crater initiated a series of ash emissions, which continued at decreasing strength for about 20 minutes. A thermal camera located at La Montagnola, 3 km S of the summit craters, showed ejected hot material and a cauliflower-shaped, dark gray ash plume that rose about 1 km and drifted E. Ashfall was reported in areas SE from Etna to Catania (27 km S). An inspection the next morning revealed that the W wall of the BN-1 crater of Bocca Nuova had collapsed. During 25-29 August a total of seven explosions were recorded.

INGV-CT reported a series of landslides on 19 June from the NW rim of the pit crater located on the lower E side of Etna's Southeast Crater. The collapses generated small ash clouds that drifted NE, and changed the pit crater's morphology. Data from a thermal camera showed thermal anomalies over 180 degrees Celsius indicating that the landslide material was hot. Fumarolic activity was seen in the landslide area during an inspection the day before.

INGV-CT reported that on 8 April a dozen low-frequency events located near the summit of Etna were detected by the seismic network. Concurrently, a dark-colored ash plume rose 1 km from a pit crater located at the E base of the Southeast Crater and drifted NE. Dark emissions from the central crater were also seen a short time later. Ashfall was reported from a few local areas. On 9 April intense gas emissions were noted at the pit crater.

INGV-CT reported that during 25-31 May the NW-SE-trending fissure E of the Etna summit craters continued (since 13 May 2008) to produce active lava flows to the N of the SE end of the fissure, along the W wall of the Valle del Bove. At least three lava flows were active. Elsewhere on the volcano, activity was restricted to degassing from the Northeast Crater, from the NW and SE Bocca Nuova vents, from the E flank of the Southeast Crater, and along summit fumarolic fields. The activity was observed directly and by utilizing surveillance cameras in Milo (about 11 km ESE).

INGV-CT reported that after a week of Strombolian activity and ash emission from the top of Etna's 2008 eruptive fissure, activity from the summit craters during 16-22 March consisted of degassing from the Northeast Crater, the NW Bocca Nuova vent, from the E flank of the Southeast Crater, and along summit fumarolic fields. The activity was observed directly and by utilizing surveillance cameras situated in Milo (about 11 km ESE). The NW-SE-trending fissure E of the summit craters continued (since 13 May 2008) to produce active lava flows to the N of the SE end of the fissure, along the W wall of the Valle del Bove.

INGV-CT reported that during 5-11 January activity from Etna's summit craters was observed utilizing surveillance cameras situated in Milo (about 11 km ESE); inclement weather prevented direct inspection of the summit area. Degassing was seen from the NW Bocca Nuova vent, from the walls and floor of Southeast Crater, and along summit fumarolic fields. The NW-SE-trending fissure E of the summit craters continued (since 13 May 2008) to produce active lava flows to the N of the SE end of the fissure, along the W wall of the Valle del Bove.

INGV-CT reported that an inspection of Etna's summit craters on 19 November revealed degassing from the Northeast Crater and, to a lesser degree, from the Bocca Nuova BN-1 crater. A 100-m-long lava flow was active in mid-November; the lava flow field on the upper Valle del Bove has been active since 13 May.

INGV-CT reported that an inspection of Etna's summit craters on 15 July revealed degassing from the Northeast Crater and to a lesser degree from the BN-1 crater of the Bocca Nuova. Explosive activity was restricted to Vent 2 of the active NW-SE-trending fissure E of the summit craters and characterized by weak Strombolian activity and diffuse ash emissions. During 15 and 17 July lava flows were active in the Valle del Bove. On 17 July, no explosive activity was seen along the fissure.

Based on observations using the summit web camera, INGV-CT reported that a week of Strombolian activity from a depression on the E flank of the South East Crater at the summit of Etna ceased on 28 April. On 1 May, a seismic swarm was detected along the NE rift and degassing from the South East Crater and the North East Crater was noted. On 10 May, an eruption produced ash plumes that drifted N; observations were hindered due to cloud cover, but the ash cloud was detected on satellite imagery. Lava flows advanced about 6.4 km E and covered the W wall of the Valle del Bove. Ashfall was reported in multiple areas during 10-11 May.

Based on information from the Istituto Nazionale di Geofisica e Vulcanologia and observations using satellite imagery, the Toulouse VAAC reported that a diffuse eruption plume from Etna rose to an altitude of 4.6 km (15,000 ft) a.s.l. during 4-5 September and drifted E.

According to news reports, a Strombolian eruption on 4 September produced lava flows that descended down the flanks. Ashfall was reported in several villages and the airport in Catania closed through the night.

On 15 August, the South East Crater at the summit of Etna produced ash emissions from a depression on the E flank. A resultant ash plume rose a few tens of meters and quickly dispersed. Ashfall formed a red deposit on the flanks of the South East Crater cone. Based on observations using the summit web camera, incandescent blocks associated with energetic emissions of ash were propelled out of the depression on 21 August. A field assessment on 22 August revealed that the ash emissions were associated with collapses within the depression. During 23-24 August, the ash emissions increased in frequency and erupted incandescent blocks were again observed.

A summit eruption that occurred at Etna on 29 April began with an increase in volcanic tremor and was followed by lava fountaining and a vertical ash plume. Lava flows traveled E within the Valle del Bove. The INGV-CT monitoring web cameras showed that the eruption lasted about 8-9 hours.

A summit eruption that occurred at Etna on 11 April began with an increase in volcanic tremor and was followed by lava fountaining. A resultant ash plume drifted E; ashfall was reported as far as Zafferana, about 10 km to the E. Two lava flows were observed at the summit of Etna, one towards the E within the Valle del Bove, and the second to the S. The E lava flow stopped 3 km away at the base of the Serra Giannicola Grande, within the W Valle del Bove. The second flow stopped near Mt. Frumento Supino (less than 1 km S of the summit). The INGV-CT monitoring web cameras showed that the eruption lasted about 5 hours.

In the morning of 29 March, an increase in volcanic tremor at Etna was accompanied by lava fountaining and an ash plume that drifted NE. Three fissures opened and produced lava flows. The first two fissures produced lava flows from vents located on the SE flank of Bocca Nuova and in the saddle between Bocca Nuova and Southeast Crater (SEC), in the same location of the October-November 2006 events. The two flows merged down slope and traveled less than 1 km S, halting at the rim of Cratere del Piano. The third fissure opened at the E base of SEC, and the lava flow spread within the upper Valle del Bove. The flows stopped by early afternoon. Ash and lapilli fallout occurred in a narrow zone between SEC, Rifugio Citelli and Giardini Naxos, on the NE flank of the volcano.

Based on increased volcanic tremor, INGV reported that weak Strombolian activity and emissions of lava and ash from the SE Crater of Etna occurred on 6 December. Later that day, seismicity dropped and explosive activity stopped. On 11 December, INGV monitoring cameras recorded Strombolian activity that resumed from a pit on the SE Crater, following a weak increase of seismicity. Lava was emitted continuously from the E flank of the SE Crater since 13 October. Lava also flowed from a vent at 2800 m elevation on the W headwall of the Valle del Bove. The eruption and lava flows from the 2800-m vent finished on 15 December. The Toulouse VAAC reported that a diffuse plume possibly containing ash was intermittently visible from a webcam on 13 December, drifting E.

INGV reported that a black ash plume from Etna rose above cloud cover to an altitude of 4.8 km (15,700 ft) a.s.l. on 21 November at about 1500. Light ashfall was reported from areas E and NE, including Rifugio Citelli (6 km NE of the SE Crater). After 1900, the cloud cover dissipated and the SE Crater came into view. Strombolian activity generated jets of material greater than 300 m high. Lava flowed down the SSE flanks and continued into 23 November. According to the Toulouse VAAC, mild eruption plumes were visible on an INGV webcam on 24 November. Due to the possible presence of ash plumes, the Fontanarossa airport in E Sicily closed from the evening of 24 November until early 28 November.

An English-language report on Etna's activity during 31 August-5 November that was recently prepared and distributed by scientists from the Instituto Nazionale di Geofisica e Volcanologia (INGV) is summarized this week.

Strombolian activity and lava flows from SE Crater that began on 31 August continued until 15 September. During 22-27 September, 3-6 October, and 10-11 October, new but similar eruptive episodes with Strombolian explosions produced lava flows.

On 12 October, a short fissure opened on the ESE flank at the base of SE Crater. Lava spread in the upper Valle del Bove and advanced a few hundred meters downslope. On 17 October, mild spattering led to the growth of three hornitos on the fissure. Vigorous Strombolian activity from a vent in the SE Crater and large explosions occurred on 20 October. Lava flowed less than 1 km SE and a new cone grew at the summit.

On 23 October, vigorous Strombolian activity and lava fountaining from SE Crater marked a new eruptive episode. Lava flowed down the ESE flank and the summit cone rapidly grew. The explosive activity ceased the next day and was followed by ash emissions. Field observations revealed that a 50 m wide collapse pit opened on the SE flank and the new cone at the summit of the SE Crater had collapsed.

On 25 October ash emissions and weak Strombolian activity were observed from the summit of the SE Crater. Lava flows were emitted from fissures on the SSE flank and the S base of the central summit cone. On 27 October, ash emissions were followed by lava flows from the SSE flank fissure. Ash emissions on 29 and 30 October produced ashfall in inhabited areas including Catania, 27 km S of the summit cone. Lava continued to flow from the 25 October fissure and from the 12 October fissure at least until 5 November, when field observers reported actively flowing lava in the uppermost portions of the flow fields.

On 14 July, a fissure opened on the E flank of the SE Crater of Etna and produced a lava flow that traveled E to the Valle del Bove. Moderate Strombolian activity from the E flank of the SE Crater produced a small amount of ashfall on Catania (~25 km SSE of the volcano). The lava flow reached a maximum distance of 3 km within the Valle del Bove and ceased on 24 July. On 26 July, strong explosions were heard from the rim of the NE crater.

On 31 August, Strombolian activity from the summit of the SE Crater produced lapilli and bombs that fell mainly in the crater. The ejecta filled the crater and overflowed on the E side on 5 September, forming lava falls that accumulated in a steep-sided circular depression on the middle part of the E flank. On 7 September, the sluggish a'a' flow breached the E rim and spread out on the E flank of the SE Crater and towards the Valle del Bove rim. Explosive activity at the SE Crater summit produced lava blocks that fell to the base of the cone.

On 10 September, a rockfall from a wall that divided the SE Crater and the depression on the middle part of the E flank produced an ash plume that drifted W. Lava flows and Strombolian activity from the summit of the SE Crater continued on 11 September.

INGV-CT reported that the effusive eruption that began at Etna on 7 September 2004 continued through 27 January 2005. On 18 January the INGV-CT web camera recorded a dense, pulsating gas plume emanating from the summit of Northeast Crater for a few minutes. During the afternoon of the 18th, a new lava flow formed upslope along the 2,620-m eruptive fissure at about 2,450 m elevation. The lava flow spread about 200 m SE along the middle wall of the western Valle del Bove. The flow moved slowly, stopping after about 24 hours. Lava emission stopped from the ephemeral vents below 2,000 m elevation. The lower ephemeral vents started to emit lava again on 19 January. During the afternoon of 22 January, two new lava flows traveled from 2,400 m elevation, along the same lava-tube system fed by the 2,620-m-elevation vent. Two parallel, fast-moving flows spread E. They were still visible on images recorded on 27 January by the INGV-CT web camera at Milo. In addition, a number of ephemeral vents and small flows at the lower end of the lava tube were visible.

On 8 January, Etna erupted plumes of black ash from its SE Crater. Following the eruption, a reddish glow emanated from the crater. Toulouse VAAC reported that the ash was barely discernable by satellite imagery.

According the Etna Volcan Sicilen website, lava emission continued at Etna during 6-11 October from vents around 2,620 m and 2,350 m elevations. Lava emission from the 2,620-m-elevation vent flowed in several arms down Valle del Bove. At the 2,350-m-elevation vent, lava flowed in tunnels for tens of meters.

News reports indicated that INGV scientists, during a helicopter survey on 15 September, noted that both of the effusive openings on the western side of the Valle del Bove had decreased in activity. The lava flow from the vent at 2,620 m a.s.l. had reached a maximum length of 1,450 m but had almost stopped, like the smaller one near Monti Centenari at 1,900 m a.s.l.

INGV-CT reported that at about 1030 on 7 September an effusive eruption began at Etna's summit. An approximately 230-m-long fissure opened at the base of Southeast Crater without any significant accompanying seismicity. Lava flowed from the base of the fissure, spreading SE towards the Valle del Bove. After 2 days of slow expansion of the field of fractures, between 0600 and 0700 on 10 September a new effusive vent opened at 2,650-m elevation on the upper western flank of the Valle del Bove. A lava flow poured out from this vent towards the E, spreading on the upper wall of the Valle del Bove. No explosive activity accompanied the emission of lava, but some phreatic explosions were triggered by lava flowing on a thick layer of snow. By 0930 the longest branch of the lava flow was 300 m long and 50 m wide. On the afternoon of 13 September, another effusive vent opened within the Valle del Bove, at about 2,200-m elevation, about 500 m SE of the previous vent. A lava flow from this vent spread in the valley at low output rate, while the previous flow from the vent at 2,650 m continued to feed a 1-km long lava flow. No lava flows threatened villages; the closest lava flow was at least 10 km from the nearest village.

INGV-CT reported that, following the last eruption of Etna on 28 January 2003, no further eruptive activity had been observed as of 8 March 2004. Summit activity was limited to pulsating gas emissions from Northeast Crater and from one of the two vents within Bocca Nuova crater. The other central crater vents and Southeast Crater were essentially blocked, producing only extremely weak gas emissions. The first significant variation from this very low level of activity occurred between 12 and 14 February 2004, when a weak ash emission was observed within the summit crater plume. The high amount of juvenile components within the ash suggested an uprise of magma into the summit feeder conduit of the volcano, the first to occur since the end of the 2002-2003 flank eruption. Since January, several shallow earthquakes have been recorded at Pernicana fault on the volcano's NE flank.

INGV-CT reported that on 11 August at 1715 an increase in volcanic tremor at the summit seismic stations on Etna lasted about 15 minutes. The tremor was followed by about 30 minutes of strong explosion earthquakes that were recorded at all INGV-CT seismic stations. This was the first such event recorded since the end of the flank eruption on 28 January 2003. A red puff of ash from Northeast Crater was visible on a INGV-CT web camera located at Milo, about 11 km from the summit. Red glow from the crater was seen during the night. No explosive activity or loud sounds occurred during a field survey on 14 August, and no explosion earthquakes or tremor were recorded during 11-16 August. Periodic measurements of the gas plume at the volcano's summit revealed that gas emissions had generally decreased since the end of the flank eruption.

As of 16 August, the mild volcanic activity that was seen at Etna's Northeast Crater on 11 August was no longer observed. No incandescence was visible during the evening. Gas emission continued from the summit craters.

According to the Etna Volcan Sicilian website, loud noises emanated from Etna's Northeast Crater on 5 and 6 August, but only strong degassing was seen. The Italy's Volcanoes website reported that on the 11th, weak, fluctuating glow was observed at the base of a dense gas column emitted from Northeast Crater. The gas column has been emitted from the crater for several months.

During the end of July, heightened seismicity occurred in the eastern area of Etna. Also, vigorous degassing continued at Northeast crater, and less intense degassing occurred at Bocca Nuova crater. Four small earthquakes with magnitudes up to 2.6 occurred on 30 July between the villages of Milo and Zafferana on the volcano's E flank.

As of 30 June strong degassing continued at Etna's Northeast Crater, and there were minor emissions at Bocca Nuova crater. A series of small, shallow earthquakes occurred on 26 June under Etna's E flank near the village of Fornazzo. These earthquakes are thought to be associated with an unstable section of the volcano that is slowly sliding eastward.

The Toulouse VAAC reported that the Etna web video camera showed an ash plume below ~4 km a.s.l. drifting SE on 7 June. According to the Italy's Volcanoes website, in early June intense gas emissions took place from Northeast Crater, often feeding a plume that extended tens of kilometers. At Bocca Nuova crater strong gas emissions and occasional strong explosions occurred, but no fresh volcanic material was ejected beyond the pit. Gas was emitted from two pits in Voragine crater. There was a progressive increase in the number and activity of fumaroles near Southeast Crater's summit and the S flank.

According to the Italy's Volcanoes website, on 12 February a series of ten earthquakes occurred on Etna's NE flank in the same area that was affected by vigorous seismicity and ground deformation before and during the initial phase of the 27 October 2002 to 28 January 2003 eruption. The largest earthquake occurred on 13 February at 0632, with a magnitude of 3.8 and a focal depth of 1.65 km.

INGV-CT reported that the Etna flank eruption that began on 27 October 2002 ended on 28 January 2003 after 3 months of activity. In January, lava flows and Strombolian explosions were confined to Etna's S flank at a 2,750-m-elevation vent. Lava flows emitted from this vent formed a fan, covering the previous lava-flow field. Strombolian activity from the 2,750-m-cinder cone significantly declined on 27 January and ended on 29 January. In addition, lava flows slowed down on the 27th and by the 29th were no longer fed and were cooling down. At the same time there was a significant decrease in SO2 emission to a minimum of 2,000 tons/day on 29 January. Volcanic-tremor amplitude also showed a marked decrease on 27 January. The end of the eruption was interpreted to occur on 28 January at 2240 when volcanic tremor decreased to background levels.

According to the Italy's Volcanoes website, the eruption that began at Etna on 26-27 October 2002 ended during 28-29 January 2003. During the previous week, volcanic activity decreased at the pyroclastic cone on Etna's upper S flank and by the evening of the 28th seismic and volcanic activity had ceased.

According to the Etna Volcan Sicilien website, during 22-27 January low-to-moderate activity continued at the 2,750-m-elevation pyroclastic cone on Etna's upper S flank. Occasional strong explosions, and gas-and-ash emissions occurred. Lava flows were observed traveling to the S and W.

According to the Etna Volcan Sicilien website, on 15 January ash emission increased at the 2,750-m-elevation pyroclastic cone on the volcano's upper S flank. There was also an associated increase in lava emission towards the S. On the 16th explosive activity continued at the 2,750-m-elevation cone, with occasional explosions ejecting volcanic material onto an area within 300 m in the crater. On the 20th a reduction in activity occurred at the pyroclastic cone in comparison to the previous day, with ten explosions occurring in 8 hours. According to the Toulouse VAAC, low-level steam-and-ash clouds were occasionally visible on the Etna web camera.

According to the Etna Volcan Sicilien website, a reduction in explosive activity at the 2,750-m pyroclastic cone on the upper S flank of the volcano occurred for about a week until 10 January. On the 10th explosions occurred every few seconds, sending volcanic material several meters high that fell within the cone. Lava flows from the cone traveled towards the S.

According to the Etna Volcan Sicilian website Strombolian activity continued at the crater at 2,750 m elevation during 31 December to 4 January. Explosive activity was accompanied by weak, sporadic ash emission. During 3-4 January the level of activity at the crater seemed to be lower than during the previous days. Lava was emitted from a fissure at the base of the 2,750-m crater that split into two arms, with both traveling S.

INGV-CT reported that the eruption that began at Etna on 27 October continued through 23 December. Lava flows and Strombolian activity occurred at the volcano's S flank at the 2,750 m vent. During 9-10 December, two vents opened at the SE base of the 2,750-m cinder cone that fed four major lava flows which spread between the S and SE. On the 17th a strong explosion occurred at a building in the Rifugio Sapienza tourist area. The explosion was not directly caused by the eruption, but by the vaporization of oil or water, still contained inside the building, when the lava flow contacted it.

The effusion rate from the two vents gradually decreased, eventually causing the closure of the western vent and then the lack of supply to the lava flows spreading SW towards Monte Nero. A new vent opened on 17 December at the S base of the 2,750 m cinder cone, a few meters W of the previous vents. A lava flow soon started from this vent, spreading SW towards Monte Nero. Lava flows from the 17 December vent slowed down and crusted over on 22 December, when a new vent opened at the SW base of the 2,750 m cinder cone. Again, a flow traveled SW towards Monte Nero. As of 23 December the lava flow continued to flow in this direction. SO2 emission rates remained relatively low, around 7,000 tons per day. Updated maps of the lava flows, and reports of the eruptive activity, gas emission, and ash composition can be found on the INGV-CT web page (in Italian).

Eruptive activity continued during 16-21 December at Etna. According to the Italy's Volcanoes website, during the evening of 16-17 December explosive activity changed from lava-and-ash fountaining to discrete, ash-poor Strombolian explosions. Lava continued to flow from two vents at the S base of the active pyroclastic cone at 2,750-m elevation. During 19-21 December there were frequent shifts from lava fountaining and ash emissions to Strombolian explosions, and vice versa. Lava was no longer traveling towards the Rifugio Sapienza tourist complex. Ash had not fallen at the Catania airport since 8 December, so the airport returned to full service.

According to an INGV-CT report, the eruption that began at Etna on 27 October continued through 13 December. On 8 December there was a sudden change in eruptive style. Strong Strombolian explosions from the 2,800-m vent replaced previous fire fountaining, signaling the end of ash emissions for about 24 hours. On 9 December fire fountaining resumed at this vent, but on the10th activity changed to Strombolian explosions. This alternating activity culminated on 10 December with the opening of two vents on the SSE base of the cone and the emission of two lava flows. These flows spread SW towards Monte Nero, and S towards the cinder cone that formed at 2,550-m elevation during the 2001 eruption (also called Laghetto cone).

The S flow expanded on the 11th and 12th, and as of the 13th it had reached 300 m from the road leading to the Rifugio Sapienza tourist area, about 3.3 km from the vent. Civil Protection authorities, for the second time during this eruption, built an earth dam to divert the lava away from Rifugio Sapienza structures. SO2 emission rates significantly decreased on 1 December, dropping from previous estimates of ~20,000 to ~7,000 tons/day.

According to a Reuters news article, a lava flow that reached the Rifugio Sapienza tourist area on the evening of 16 December caused an explosion that destroyed a building and injured 32 people. The article stated that the exact cause of the explosion was unknown.

According to the Italy's Volcanoes website, as of 5 December explosive activity continued at one of Etna's vents in the northernmost portion of the fissure that opened on Etna's upper S flank on 27 October. Ash emission and lava fountaining were vigorous, with continuous tephra fall affecting areas NE and E of Etna during several days before 5 December. Lava emission seemed to temporarily cease after about 3 weeks of near-continuous activity from vents on the flanks of the new pyroclastic-cone complex. Intermittent seismicity occurred at and around Etna. The largest reported damage occurred at a vacated school building near Giarre that partially collapsed. On 8 December ash emission and lava fountaining at the 2,800-m vent changed to violent Strombolian explosions. The following day ash emission recommenced, and on 10 December explosive activity shifted to the main pyroclastic cone, while the 2,800-m cone became less active, and lava was emitted that fed a flow to the S. By the 11th the lava flow had divided into several branches, with one slowly advancing toward the Rifugio Sapienza tourist area.

Etna continued to erupt through 2 December. According to the Italy's Volcanoes website, during the afternoon on 1 December ash fell in Catania and surrounding areas, leading to the closure of Fontanarossa Airport. On the 2nd, explosive activity continued at two vents on Etna's upper S flank and lava was emitted from a third vent at the SW base of the large pyroclastic cone that formed during the first 5 weeks of activity. In addition, the most advanced part of the most recently active lava flow that burned part of a forest on the SW flank seemed to have stopped. According to the Toulouse VAAC, since the eruption began on 27 October there has been unsteady activity at the volcano with periods of stronger activity leading to temporary ash emission.

The 2002 Mt. Etna flank eruption that began on 27th October is continuing, after almost a month of activity. During this period several distinct phases of eruptive style have been observed. The first phase of the eruption ended on 5th November, when lava flows from the northern fissure stopped. Strombolian and fire-fountaining activity continued at the southern fissure, localised within the 2750 m elevation cinder cone that formed during early November. Lava jets reached heights of over 300 m above the crater, forming an ash column that spread mostly N, due to the strong wind, and reached an elevation of 4.7 km a.s.l..

The second phase of activity started on 12th November, when strong jets and continuous emission of ash gave way suddenly to mild Strombolian activity. Lava flows began to spread SW from the 2750 m vent on the 13th November. These flows ran parallel to the October flows towards Monte Nero and achieved a maximum length of 4 km on 19th November, stopping just 300 m before Casa Santa Barbara, at 1770 m a.s.l.. Lava output from the main vent then declined, and overflows covered the previous flow channel. As of 25th November the most advanced active flow fronts were located within 1 km from the vent.

Between 20 and 21st November another new vent opened on the SSE base of the 2750 m cinder cone. This vent produced a new lava flow that spread south towards Rifugio Sapienza. The flow length reached 1.9 km on the 22nd, and 2.7 km on the 23rd, covering the Rifugio K. The Rifugio Sapienza was threatened by the flow, and Civil Protection soon built up two earth barriers to divert the lava towards the east of buildings, as in the 2001 eruption. This diversion was once again successful, and the flow eventually stopped on 24th, a few metres before reaching the SP92 road connecting Zafferana to Rifugio Sapienza, after having traveled 3.6 km from the main vent.

Early on the 25th November, two new explosive vents opened to the N and SSE of the 2750 m cinder cone. This caused a shift in explosive activity from the crater of the previous cone to the newly formed vents, which produced fire fountaining activity and an ash plume rising to 4.7 km elevation and spreading north. Immediately afterwards the effusion rate of the south lava flow (towards Rifugio Sapienza) significantly decreased, and slightly increased in the southwest flow (towards Casa Santa Barbara). This caused new overflows above the previous flow channel on the lava flows directed to Casa Santa Barbara. The low effusion rate did not allow these flows to reach previous flow lengths, and they were less than 1 km long when last observed on 25th November. Observation of the flow field was impossible on the 26 November due to poor weather conditions.

SO2 emission from the volcano is still very high and fluctuating, keeping within the range of 20,000 to 7,000 tons per day. Taken together, the high amount of gas released by the volcano, the still high volcanic tremor, and the explosive activity still ongoing at the south vent, rather suggest that the eruption may continue for some time.

The eruption that began at Etna on 27 October continued through 12 November. INGV-CT reported that on 12 November at 1340 volcanic tremor recorded by their seismic network gradually increased, reaching an amplitude two times higher than before. This seismicity increase occurred when fire fountaining and ash emission from the vent at 2,750 m elevation on the volcano's S flank suddenly stopped and were substituted by Strombolian activity. This change marked an increase in the magma level within the conduit, and on the 13th at 1600 a lava flow was emitted from the S base of the upper cinder cone that formed around the 2,750-m vent. The lava flow spread S, filling up the intermediate and lower cinder cones along the same eruptive fissure. It expanded SW towards Monte Nero, running parallel to the lava flow that had stopped on 31 October. By 14 November at 0930 the lava-flow front had reached 1.2 km in length, was at an elevation of 2,450 m a.s.l., and traveled at a velocity of about 2-3 m per minute. Strombolian activity was substituted by fire fountaining and the emission of ash that reached 3.5 km a.s.l. and drifted N and E. SO2 emission from Etna remained very high and fluctuated around 20,000 tons per day. According to a news article, Fontanarossa Airport in Catania, which had been closed since 10 November, reopened on the 13th. The Toulouse VAAC reported that moderate-to-severe ash emissions had occurred at Etna since the eruption began, but had become weaker since the afternoon of 12 November.

An eruption began at Etna on 27 October. According to a report from INGV-CT, the lava supply from Etna's main vents ended by 3 November. At that time lava flows were no longer emitted from the volcano's S and N flanks. As of the 11th, fire fountaining continued from the S vent at 2,750 m elevation, near Torre del Filosofo. All data (gas emission, volcanic tremor, composition of the ash) suggest a steady state at this vent. Ash fallout caused intermittent disruption at the Catania airport and damage to buildings. The Toulouse VAAC reported that moderate-to-severe ash emissions had occurred since the eruption began. During 6-12 November ash clouds rose to a maximum height of ~6.1 km a.s.l.

Volcanic and seismic activity continued at Etna during 29 October to 4 November after a new eruption commenced on 27 October. The eruption consisted of fissures opening on the volcano's S and N flanks, lava emission from these fissures, and fire fountains rising several hundreds of meters. Significant ash emissions led to the closure of Fontanarossa airport and produced ash clouds that drifted as far as Libya. According to an INGV-CT report, two major lava flows were emitted from the lower end of the northern fissure and spread toward the NE and E. The NE flow stopped on 31 October after traveling 2 km, when a decline in effusion rate was observed. As of 1 November the E flow had slowed down, but it was still moving and crusting over in the middle portion of the flow field. A few sectors of solid crust were detected during a survey with the helicopter of the Civil Protection using a thermal camera. This suggests that a lava tube is forming on this lava flow. The lava flow from the S fissure started about 12 hours after the N one, spread SW, and split in two branches around Monte Nero, following the same path as one of the 2001 eruption's lava branches. The S flows stopped on 31 October, reaching a total length of about 1 km. Fire fountains and phreatomagmatic activity decreased in intensity with time at both the N and S fissures. As of 1 November the effusion rate from the N fissure was declining, which increased the possibility of lava-tube formation along the E flow.

According to the Toulouse VAAC, ash was emitted from Etna during the report period and was occasionally visible on satellite imagery rising to 6.1 km a.s.l. On 1 November ash was visible on satellite imagery extending ENE from Etna's summit, reaching the coast of Greece. On several occasions meteorological clouds in the vicinity of Etna prohibited satellite views, but the Etna web video camera located in Catania showed continuous ash emission on 4 November.

A relatively large eruption began at Etna on 27 October, following a series of ~200 small earthquakes the previous evening. The eruption began with fissures opening on the volcano's S and NE slopes around 2,700 m elevation, between Southeast Crater and Montagnola cone. Lava fountains rose 100-200 m, lava flows were emitted from the fissures, and significant ash plumes were produced. On the 28th seismicity continued, with a M 3.8 earthquake occurring beneath the volcano. Lava flows cut across the road connecting the towns of Linguaglossa and Piano Provenzana and lava ignited several forest fires near Piano Provenzana. The lava flows were estimated to be 365 m wide and 6.1 m high.

Ditches were dug in an effort to control lava flows, but by the 29th they were completely covered by lava. Authorities also tried to control the flows by having planes douse the lava with water, causing the flows to cool and stagnate, but they continued to travel down the volcano's flanks. Authorities stressed that the popular ski town of Linguaglossa (6,000 residents), located ~15 km NE of Etna's summit, was not in danger of being engulfed by lava flows. As a precautionary measure ~50 residents were evacuated and schools were closed. By the 28th lava flows had destroyed several hotels, restaurants, a ski school, ski lift pylons, and power lines on the volcano's flanks. Ash fell in towns at the base of the volcano. Some streets in the town of Nicolosi, ~15 km S of the summit, were covered with a 5-cm-thick layer of ash.

During 27 to at least 29 October ash clouds were visible on satellite imagery and the Etna volcano video camera, reaching a maximum height of ~6.4 km a.s.l. The clouds drifted towards the SE and on the 27th one had reached ~350 km S to Libya. From the beginning of the eruption, Catania's Fontanarossa airport was closed. Ash emissions continued on the 29th and a M 4.4 earthquake occurred around 1100, damaging hundreds of buildings in the town of Santa Venerina on the volcano's SE flank.

On 17 October around 1530, a significant amount of brown ash was emitted from Etna's Northeast Crater. No activity was visible during the night from cities at the base of the volcano. Earthquakes with magnitudes between 2.3 and 2.7 occurred on the morning of the 18th during 0427-0500. Approximately 30 minutes later the Northeast Crater expelled plumes of brown ash.

When weather conditions were favorable on 22 and 27 September, black ash was seen rising from Etna's Northeast Crater. On the 27th ash was also emitted from Bocca Nuova crater. On 1 October degassing was seen at Bocca Nuova every 5-10 minutes, lasting ~30 seconds. No ash was emitted. Poor weather conditions prevented observations at the other craters.

During 9-14 September, Strombolian activity continued at Etna's Northeast Crater, with ash and incandescent material being ejected from it. Volcanic bombs ejected vertically to heights of 100-150 m fell within the crater. Low-level ash clouds were visible on satellite imagery.

On 4 September the strong ash emissions that occurred at Etna's Northeast Crater the previous week were no longer observed, and only degassing occurred. Ash was emitted from Bocca Nuova crater once every 5-10 minutes and gas was emitted from Voragine and Southeast craters. On the night of 6 September, a M 5.6 earthquake, centered near the town of Palermo (~145 km NW of Etna) produced aftershocks that were felt in Catania (25 km SSE of Etna). On 8 September, strong Strombolian activity was observed at Northeast Crater. Explosions occurred every 1-3 minutes and incandescent material was ejected outside of the crater.

The Toulouse VAAC reported that beginning on 18 August around 1030, a web camera revealed that ash was near Etna's summit. Ash was also visible on satellite imagery. The Etna Volcan Sicilian website reported that during a visit to Etna on 18 August, ash was emitted from Northeast Crater, and gas, ash, and blocks were emitted from Bocca Nuova crater. Only gas was emitted from Voragine and Southeast craters.

The Toulouse VAAC reported that beginning on 9 August at 0630 ash was visible at Etna on a web cam. The ash was around summit level and was not visible on satellite imagery. According to the Etna Volcan Sicilian website, during a visit to the summit on 8 August brown ash was emitted from Bocca Nuova crater. By the 9th only gas was released from Bocca Nuova.

Activity continued at Etna during 31 July-6 August with brown ash explosions from Bocca Nuova crater. SO2 emissions were detected from Voragine crater on 4 August. Strong explosions originating in Northeast Crater were felt during the afternoon of 6 August. The area near Northeast Crater contained numerous bombs, some more than 70 cm in size weighing more than 30 kg.

During a visit to Etna on 26 July it was revealed that Strombolian activity had continued at Northeast Crater. Activity consisted of volcanic bombs being thrown beyond the rim of the crater. There was very little activity at Voragine and Southeast craters. According to the Toulouse VAAC, ash was observed on 29 July at 0855, possibly around summit level.

During 19-21 July, explosions at Etna's Northeast Crater emitted gas and ash, and ejected volcanic bombs and fresh lava that landed within the crater. At Bocca Nuova crater, gas-and-ash emissions and explosions also occurred. At Voragine crater, degassing was less intense than it had been during previous observations on 22 June 2002. According to the Air Force Weather Agency, surface observations revealed that on 22 July a low-level ash cloud was visible. Ash was not visible on satellite imagery.

During 17-23 June, ash continued to be emitted from Etna's Northeast Crater and to a lesser extent from Bocca Nuova crater. Ash plumes from Northeast Crater rose several hundred meters. Quiet degassing occurred at Voragine and Southeast craters. Seismicity was at low levels during the week.

On 13 June ash emissions, which began several days earlier, intensified at Etna's Northeast Crater. A slight increase in seismicity occurred at the same time, with a few events being felt by residents on the volcano's S and SE flanks. The Toulouse VAAC reported that volcanic ash was observed on satellite imagery on 14 June at 0450 drifting SSW at heights of ~2.4-4.6 km a.s.l. Beginning on 16 June ash was emitted from Bocca Nuova crater as well. No incandescence was visible at the volcano during the night.

During late May, volcanic and seismic activity were low at Etna. Volcanism consisted of gas emissions from Northeast, Voragine, and Bocca Nuova craters. Also, several small new fumaroles formed on the lower SE flank of Southeast Crater during about 26 May to 2 June. According to Italy's Volcanoes website, a visit to the summit craters on 30 May revealed that few changes had occurred there since the previous visit on 28 January.

Ash emissions that had been occurring at Bocca Nuova crater for several weeks seemed to diminish during 23-25 April. During the 24th-25th, only ash-free gas emissions were observed, mostly from Bocca Nuova and Northeast craters. On the 28th a brief emission of brown ash was seen coming from the W side of Bocca Nuova. No felt earthquakes occurred after the 15th.

During 14-18 April, a dense ash plume persisted at the SE vent of Etna's Bocca Nuova crater. Dense ash-free plumes rose above Voragine and Northeast craters. Earthquakes continued on Etna's SE flank, and there were reports of seismicity on the NE flank.

Signs of unrest persisted at Etna during 6-14 April. On 13 April two earthquakes, with magnitudes 2.7-3, were felt by the local population. They occurred on the SE flank between the towns of Zafferana and Santa Venerina. During the report period, ash emissions were nearly continuous at Bocca Nuova crater, rising to several hundreds of meters. No incandescence was visible in the crater.

Ash emissions during 29 March-6 April at Etna's Bocca Nuova crater deposited ash as far as the town of Catania ~25 km SSE of the volcano. Ash emissions apparently stopped at Northeast Crater by 2 April; only steam was visible rising above the crater. During 4 and 5 April, earthquakes occurred at or near Etna with magnitudes up to 4.2.

Following almost 3 weeks of ash emissions from Etna's Bocca Nuova crater, around 25 March ash emissions began at Northeast Crater. The emissions appeared to coincide with a series of small earthquakes under the volcano's SE flank the night of 24-25 March. On the 27th and 28th ash from both craters rose in distinct puffs several hundreds of meters above the summit. On 28 March a small amount of ash fell on Etna's S flank, reaching as far as the town of Catania. The following day dark ash rose nearly continuously to several hundreds of meters above the summit. The Toulouse VAAC reported that no ash was visible on satellite imagery.

The emission of pink, lithic ash from Bocca Nuova crater that reportedly began on 9 March continued through 21 March. In addition, voluminous degassing occurred at Northeast Crater and there was minor fumarolic activity at Voragine and Southeast craters. No incandescence was visible at night.

Beginning around 9 March, near continuous, pulsating emissions of lithic ash were erupted from the NW vent of Bocca Nuova crater and an ash plume drifted dozens of km. The source vent of this activity was also the site of deep-seated explosions during the past 6 months. According to the Italy's Volcanoes website, the emissions might be caused by collapses within the conduit, which are thought to have occurred repeatedly following the July-August 2001 eruption.

As of 7 March there had been no new eruptive activity at Etna for several months. Numerous small earthquakes were recorded beneath the volcano's S flank, where the largest of the July-August 2001 lava flows vented. Some of the earthquakes were felt by the population near the volcano.

On 30 January low levels of activity were observed at Etna. Loud explosions occurred approximately every 5-30 minutes in the NW pit of Bocca Nuova crater, but no solid material was ejected. Degassing occurred at Bocca Nuova, Voragine, Northeast, and Southeast craters.

In mid-January, increased gas emissions occurred at Southeast Crater, and increased gas-and-ash emissions occurred at Bocca Nuova crater. Very small amounts of ash fell on the E side of the volcano, as far as the town of Acireale, ~20 km SE of the volcano. No incandescent ejections were visible at night.

Volcanic activity has been low at Etna since the voluminous lava outburst during the July-August 2001 eruptive event. On 6 January low-level fumarolic activity occurred from the western rim of Southeast Crater at intervals of a few minutes. In addition, a dense gas plume was emitted from Bocca Nuova crater. The activity at both craters continued through at least 7 January.

As of 16 October moderate amounts of degassing took place at Etna's summit craters, with most occurring at Northeast Crater. A portion of the western crater rim of the scoria cone that formed during the July-August 2000 eruption at 2,100 m elevation collapsed into the vent.

During 4 and 5 October a large amount of degassing occurred at Northeast Crater. Degassing at Bocca Nuova crater was occasionally accompanied by ash emissions generated by the crumbling of the crater walls. Gas and "smoke" were continuously emitted from the edges of the new crater located at 2.55 km elevation.

According to reports on 28 September, gas emissions from Bocca Nuova crater had been vigorous at times during the previous few weeks. In addition, a dense gas plume was visible rising above Northeast Crater. The cones that formed during the July-August 2001 eruption on the S flank continued to emit heat and minor amounts of gas.

Volcanic activity resumed at Etna's summit on 11 September. Loud explosions emanated from the NW vent of Bocca Nuova crater at 5- to 10-minute intervals. The explosions were followed by dense gas clouds, but no volcanic fragments were ejected.

During the week volcanic activity was relatively low at Etna, with no explosions or lava flows. There were several small earthquakes, and slight degassing at fissures on the volcano's flanks. Vapor was emitted from the summit craters, and was accompanied by occasional plumes of brown ash at Bocca Nuova crater.

As of 13 August lava flows appeared to cease advancing down the slopes of Etna. Slight degassing occurred from many of the fissures and the rate of SO2 emission was low in comparison to average values for the season.

News articles stated that around 1 August lava stopped advancing towards the Rifugio Sapienza tourist area, although lava in other parts of the volcano slowly continued to advance. According to the Toulouse VAAC, narrow ash clouds that rose to below 5.5 km a.s.l. were occasionally visible in satellite imagery and on Sistema Poseidon's web cam. The international Fontanarossa airport in Catania was closed, for the fourth time since the eruptive period began, during 2-5 August due to ash clouds in the area. The amount of ash emitted from the volcano decreased on 5 August and only steam with small amounts of ash located close to the ground was visible on the web cam.

During 25-31 July the eruption that began on 17 July at Etna's Southeast Crater continued at the same five eruptive fissures as the previous week. One of the fissures was located on the NE flank and the other four on the S flank at elevations of 2,950, 2,700, 2,500, and 2,100 m. During 24-26 July eruptive activity declined at the 2,100 m fissure, but then intensified on 27 July to reach the original level of Strombolian activity on the evening of 28 July. On 26 July modest lava emission occurred at the NE-flank fissure. At the 2,500 m fissure, where the strongest explosive activity occurred, a change was noted from the mostly phreatomagmatic eruptions that were prominent during the previous week to more violent Strombolian explosions and lava effusion. After lava began to flow from the 2,500 m fissure on 25 July, a pyroclastic cone began to grow around three of the vents and by 30 July the cone was ~100 m high. At the 2,950 m fissure (near the base of Southeast Crater) and 2,700 m fissure lava emission and mild explosive activity continued.

On 26 July lava from the 2,500 m elevation fissure continued to flow towards Rifugio Sapienza tourist complex, and as of 31 July a cable car base station and a small tourist shop had been destroyed by lava that surpassed constructed earth barriers. Lava continued to flow from the lower vents of the 2,100 m fissure in the direction of the town of Nicolosi (~15 km SSE of the volcano), but it was no longer considered a significant threat to the town. Near-continuous ashfall occurred S of the volcano, including in the town of Catania, ~25 km SSE of the volcano. The international Fontanarossa airport in Catania was closed repeatedly on 29 and 30 July due to ash on the runways. The Toulouse VAAC reported that ash clouds were occasionally visible on the Sistema Poseidon web cam and satellite imagery, with the highest cloud rising ~5.5 km above the volcano.

A large flank eruption began on 17 July at Etna and produced several lava flows that were emitted from four new fissures and strong explosive activity at a fifth. Four of the fissures were on the SE flank, and the fifth was on the NE flank. There were fears that lava flows from two of the fissures would reach the town of Nicolosi (~15 km SSE of the volcano) and a nearby popular tourist area. According to the Italy's Volcanoes website, the 17th eruptive episode in 2001 began on the morning of 17 July and a few hours later a new eruptive fissure opened at the S base of the Southeast Crater cone (see map of fissure locations). Mild Strombolian activity occurred from the fissure and a lava flow extended SSE. During the evening of the 17th a second eruptive fissure emitted an extensive lava flow that spread SE toward the Valle del Bove rim. On 18 July at about 0200 a seismic swarm was accompanied by the opening of a third eruptive fissure at about 2,100 m elevation. Mild Strombolian activity and a sluggish lava flow traveled toward the S. Later in the day the lava flow crossed the main access road to the S flank of Etna and headed towards Nicolosi. A spectator was seriously injured when he fell while trying to avoid projectiles.

On the evening of 18 July the fourth eruptive vent since the episode began opened near 2,700 m elevation on the SE flank. The main explosive activity occurred at this vent, including powerful Strombolian blasts that sent incandescent volcanic bombs as high as 200 m and produced ash columns that rose several kilometers. Lava from this vent progressed towards the tourist complex around the Rifugio Sapienza. On 20 July around 1100 a fifth eruptive fissure became active, but unlike the other fissures it was on the NE flank in the Valle de Leone. Lava emitted from this fissure flowed SE.

By 22 July the lava flow from the third fissure was 4 km away from Nicolosi, but it was advancing very slowly over nearly flat terrain and appeared to have stopped by the next day. Earth barriers were created in an attempt to divert lava from the tourist complex that had already been damaged by volcanic bombs. Continuous ashfall occurred near the explosive fourth vent. The entire area between the towns of Giarre (~17 km E of the volcano) and Catania (~25 km SSE of the volcano) was covered by a thin layer of ash; there was an especially large amount of ash in Catania. The Fontanarossa International Airport of Catania was closed on 22 July and again the next day due to ashfall.

The Toulouse VAAC reported that the new Etna Sistema Poseidon webcam showed ash emission starting on 20 July. The previous webcam had been damaged by earthquakes near the start of the eruption. SE-drifting ash clouds were detected several times on satellite imagery. An ash cloud was reported to have reached a maximum height of ~5.2 km on 22 July. According to news reports, a state of emergency was declared for the area near Etna on 20 July. As of 24 July the eruption was continuing.

On 13 July the 16th eruptive episode in 2001 took place at the NNE vent of Etna's Southeast Crater. During the 4-hour-long episode, lava flows, Strombolian activity, and several earthquakes occurred. The largest two earthquakes had magnitudes of 3.9 and were felt 15 km away in the town of Nicolosi. The Toulouse VAAC reported that a small ash cloud was visible on the Sistema Poseidon Etna webcam and on satellite imagery. It did not rise above 3.7 km a.s.l.

After 4.5 days of low-level activity at Etna, an eruption occurred at Southeast Crater on 4 July that lasted for approximately 5 hours. The episode began with lava flowing from the NNE vent towards the NE and SSE and was followed by modest Strombolian activity. At the summit vent powerful explosions sent an incandescent fountain up to 400-500 m high and several large magma bubbles burst sending fragments to the base of the Southeast Crater cone. A dense tephra column rose from the summit vent and deposited ash on Etna's SE flank. Fine ash and 3- to 5-mm-long Pele's hair fell as far as the town of Acireale, ~20 km SE of the volcano. On 7 July another eruptive episode lasted for approximately 1 hour at Southeast Crater. The episode consisted of lava flows and the eruption of black ash and small volcanic blocks that reached a height of ~1 km above the volcano and drifted to the E.

An eruptive episode began on the evening of 27 June at the NNE vent on Southeast Crater. The episode began with lava flowing from the NNE vent and was followed by Strombolian activity at the NNE vent and the summit. Volcanic material was ejected to a maximum height of 400 m. No sustained lava fountains developed. The Toulouse VAAC reported that Sistema Poseidon's Etna webcam recorded renewed volcanic activity on 28 June at 0030 and associated steam and ash that did not rise far above the summit. The eleventh eruptive episode in one month began on 30 June. Inclement weather inhibited visual observations, but during a break in cloud cover around 0400 mild Strombolian activity was observed. Seismic data revealed that the episode ended around 1600. On 3 July increased degassing was observed at Northeast Crater.

During the week two eruptive episodes occurred at Etna's Southeast Crater. On 22 June an eruption began after 3 days of low activity. The eruption was similar to the previous episodes, with lava flowing down the flanks of the volcano prior to Strombolian activity. Volcanic bombs were thrown 300-400 m above the crater and lava fountains reached a maximum height of 150 m. An ash plume rose up to 3 km above the crater. The Toulouse VAAC reported that the ash plume was visible on Sistema Poseidon's Etna webcam for ~2 hours, but not on satellite imagery. The same day very strong degassing was observed at Bocca Nuova crater. On 24 June another eruptive episode lasted more than 2 hours.

The Italy's Volcanoes website reported that on 13 June, after ~44 hours of low activity, the fourth eruptive episode within in a week began at Southeast Crater. The episode lasted longer and was more intense than the previous three episodes. Lava flowed from a vent on the NNE flank of Southeast Crater cone. During the most intense phase of the eruption lava fountains rose 150-200 m above the NNE flank vent. Strombolian bursts occurred so frequently that they eventually blended into one continuous pulsating fountain that rose up to 400 m. Also bursts periodically sent bombs up to 500 m above the crater rim. A small amount of ash was emitted with many of the stronger bursts. On 15 June another eruptive episode occurred with activity similar to the 13 June episode.

According the Italy's Volcanoes website, Etna's Southeast Crater was inactive on 6 June, but the following day volcanic activity commenced with lava flowing from the NNE side of the Southeast Crater cone and Strombolian bursts from the crater's summit vent. By 8 June volcanic activity decreased. A new eruptive episode began at Southeast Crater on 9 June that consisted of lava flows and Hawaiian-style lava fountaining from the NNE flank vent, and Strombolian bursts from the summit vent. Volcanic activity decreased until 11 June when Southeast Crater erupted again with more intense Strombolian activity at the summit vent than in the previous episode, and mild Strombolian activity at the NNE flank vent. A dark, tephra-laden cloud was observed rising from the summit vent, while lava fountains rose ~150 m above the NNE flank vent. The Toulouse VAAC reported that weak volcanic activity was visible on Etna WebCam imagery during 0445-0515. By 12 June no volcanic activity was observed at Southeast Crater.

According to the Italy's Volcanoes website, volcanic activity was at similar levels as it has been during the previous few weeks. Lava continued to travel from a vent on the NNE flank of the Southeast Crater cone, and mild, discontinuous Strombolian activity continued at the Southeast Crater's summit vent. Scientists determined that the lava effusion rate was approximately 5-10 cubic meters per second, which is high for Etna. On 31 May mountain guides reported that pressure waves, which were caused by explosions, were observed approximately every 10 minutes and that volcanic bombs were thrown ~100 m above the crater rim. Degassing was observed at Southeast Crater and occurred to a lesser extent at Bocca Nuova crater, but increased at Northeast Crater.

According to the Italy's Volcanoes website, through 25 May at Southeast Crater lava continued to flow from a small cone on its NNE flank and mild Strombolian activity continued at its summit vent. Volcanic activity had been relatively regular since a strong eruptive episode began on 9 May.

According to the Italy's Volcanoes website, mild eruptive activity continued at Southeast Crater, with persistent lava outflow from a vent on the NNE flank of its cone. Very weak Strombolian bursts occurred at the summit vent of the cone that occasionally sent incandescent bombs up to 100 m above the crater rim.

A Spanish tourist who visited the volcano alone has been missing since 14 May and is presumed dead. The remains of her tent were found on 18 May near the rim of Bocca Nuova crater. Local press sources reported that a rescue team found footprints leading from the tent to the rim of one of the two active pits within the crater, but no prints were found leading back from the pit. It is possible that the tourist was standing on the rim of the pit when a portion of it broke loose.

The Italy's Volcanoes website reported that after more than 8 months of minor activity (slow lava flows, degassing, light ash emission, and low-level Strombolian activity), a new episode of vigorous volcanic activity began at Southeast Crater on 9 May. On 6 May active lava flows and explosions were observed launching pyroclasts and lithics onto the volcano's S flank every 7-10 seconds. On 9 May an obvious increase in activity occurred, with Strombolian bursts occurring every few seconds. By 1745 activity further increased and lava fountains rose up to 100 m above the NNE flank fissure while a dense eruption cloud simultaneously rose above the summit vent. Local press sources reported that air traffic was rerouted during the activity. The high level of activity continued at Southeast Crater through at least 14 May and strong degassing occurred at Bocca Nuova crater.

Sistema Poseidon reported that during 30 April to 6 May degassing occurred at Bocca Nuova, Southeast, and Voragine craters. In addition to degassing, ash was emitted from Bocca Nuova crater, although after 3 May the emissions became more sporadic. Lava continued to flow from the northern vent on Southeast Crater and hornitos were formed in the area.

According to the Italy's Volcanoes website, activity at Etna's summit craters progressively increased beginning on about 20 April. Lava effusion from a vent on the lower NNE flank of Southeast Crater cone continued, with an increase in the effusion rate on 26 April. Ash vented from Bocca Nuova and light gas was emitted from a vent on the NNE flank of Southeast Crater.

Sistema Poseidon reported that during 9-15 April volcanic activity at Etna was similar to the previous week. Strombolian activity continued at Bocca Nuova crater, although by the end of the report period activity decreased in comparison to previous weeks. Lava also continued to flow down the NE flank of Southeast Crater, and degassing continued at Voragine and Northeast craters. The Australian Broadcasting Corporation reported that on about 22 April thirty small earthquakes occurred near Etna, but did not cause any injuries or property damage.

Sistema Poseidon reported that during 2-8 April Strombolian activity continued at Bucca Nuova crater. In addition, degassing of the crater was accompanied by ash emissions. Lava continued to flow from a fissure on the NE flank of Southeast Crater. Lava also flowed from a 6-m-high tumulus that formed on the N flank of the volcano at an elevation of approximately 3,095 m. Voragine and Northeast craters continued degassing at low levels.

According to the Italy's Volcanoes website, volcanic activity at Etna decreased slightly during the week ending on 6 April. The Bocca Nuova crater produced sporadic ash emissions, rather than the Strombolian activity that was observed in March 2001. Lava continued to flow from a vent low on the NNE flank of the Southeast Crater cone, as it has since approximately 20 January 2001.

According to the Italy's Volcanoes website, volcanic activity continued at Etna through the end of March. Continuous Strombolian activity was observed at a pit in Bocca Nuova Crater. Lava continued to slowly flow from the NNE flank of the Southeast Crater.

According to the Italy's Volcanoes website, during the first half of March mild, but occasionally vigorous eruptive activity continued at Etna's summit craters much like it has since mid January. Near-continuous Strombolian activity continued at two vents in Bocca Nuova Crater and Strombolian activity intermittently occurred within the central pit of the Northeast Crater. Short lobes of lava continued to form after slowly emerging from a vent on the NNE flank of the Southeast Crater cone.

According to the Italy's Volcanoes website, during late February fluctuating eruptive activity continued at Etna's summit craters. Strombolian activity continued at two vents in Bocca Nuova Crater and within the central pit of Northeast Crater. In addition, lava continued to slowly flow from the NNE flank of the Southeast Crater.

According to the Italy's Volcanoes website, on 20 February slow lava extrusion continued from a vent on the NNE flank of the Southeast Crater cone. Mild eruptive activity continued from the Bocca Nuova Crater, with Strombolian bursts from two vents on the NW and E parts of the crater. During 18-20 February, a slight increase in activity was observed at the Bocca Nuova Crater.

According to the Stromboli On-Line website, on 12 February a Stromboli On-Line research group observed lava flowing from the Southeast Crater's NE fissure as it has since late January. Lava and gas were emitted from a small tumulus in strong pulses and flowed several hundred meters into Valle Leone. On 14 February Voragine and Northeast craters were observed emitting steam and Strombolian activity occurred at both of Bocca Nuova Crater's vents. The Strombolian activity at the Bocca Nuova's NW vent was irregular and occasionally reached at least the height of the crater rim. Strombolian activity at the Bocca Nuova's SE vent was more vigorous with eruptions every few seconds. The explosions were directed roughly vertically and volcanic bombs reached up to several hundred meters above the crater rim.

According to the Italy's Volcanoes website, since 20 January lava has continuously issued at a low but persistent rate from a vent on the NNE flank of the Southeast Crater. The lava formed a small field of overlapping and adjacent flows that extend a few hundred meters. At Bocca Nuova Crater Strombolian activity increased, with ejecta being thrown above the crater rim.

Sistema Poseidon reported that during 22-28 January, eruptive activity at Etna was dominated by modest, intermittent lava flows that were emitted from the radial fracture on the N flank of the Southeast Crater. The Bocca Nuova Crater mostly produced ash-and-gas emissions during the beginning of the report period, but they decreased near the end of the period. At night, weak Strombolian activity was observed inside the crater. Limited gas emissions were released from the Northeast and Voragine craters.

According to Italy's Volcanoes website, volcanic activity increased at Etna. On 15 January intense degassing occurred at the Bocca Nuova and SE craters. On 16 January, weak Strombolian bursts occurred at 5- to 10-minute intervals at the summit vent of the SE Crater and strong gas emissions with occasional ash were released from Bocca Nuova Crater. On 21 and 22 January an active lava flow was visible extending from a fissure on the N flank of SE Crater, but no explosive activity was observed.

There has been no new eruptive activity at the summit craters of Etna since early December. Beginning on 7 January 2001, degassing from Bocca Nuova became more rhythmic, indicating some deep explosive activity. On 11 January gas emissions from the Bocca Nuova became more intense and came in distinct puffs. Southeast Crater continued to emit heat; an incandescent fumarole remained high on its SE flank, and snow rapidly melted on the S and E flanks of the cone. During the early morning of 9 January 2001, significant seismicity affected the SE flank, shaking villages and towns including Catania. Tens of thousands of people were woken by the strongest event (M 3.5, MM V). The seismic crisis ended that same day after more than 50 shocks had been registered by the monitoring network. At least three of the earthquakes measured M 3.0 or more, and several were felt by the population of a large area between Nicolosi, Fornazzo, and Catania. Cracks opened in the walls of numerous buildings, but only one uninhabited building in Zafferana partially collapsed.

According to Italy's Volcanoes website, lava emission continued from both vents in the Bocca Nuova Crater with variable intensity. On 30 November observations revealed that after 3 months of low activity a small lava flow issued from the NNE fissure in the SE Crater. Lava extrusion was accompanied by strong degassing, but there was no explosive activity.

According to the Italy's Volcanoes website, Italian press sources reported that a new burst of seismic activity occurred near Etna's NNW flank on 19-21 November. The earthquakes were thought to be of tectonic origin. During the week mild eruptive activity continued at the Bocca Nuova crater as it has for several weeks, and incandescence was noted at the NE and SE Craters.

According to the Italy's Volcanoes website, during the week mild Strombolian activity continued at the Bocca Nuova Crater. The overall level of activity appeared to be generally lower than the previous week.

According to the Italy's Volcanoes website, volcanic activity continued to fluctuate at Bocca Nuova Crater, varying from explosions to effusive lava flows as it has for several weeks. The Bocca Nuova Crater is Etna's southwestern-most crater and explosive activity was observed at its NW and E vents. In addition, incandescence was visible on the upper SE flank of the Southeast Crater cone. In related news, at 1826 on 5 November a M 3.6 earthquake occurred in eastern Sicily. The earthquake's epicenter was located near Monte Vetore, a prehistoric pyroclastic cone on Etna's S flank. In press reports Poseidon scientists interpreted the earthquake as being purely tectonic.

Information is preliminary and subject to change. All times are local (unless otherwise noted)

June 1968 (CSLP 11-68)

Eruption accompanied by two sharp earth tremors, clouds of dense black smoke. Additional information obtained from UPI dispatch, 1948 EST: Streams of red-hot lava on Etna for last two months. Today's tremors registered II on XII-point Mercalli scale (with IV registered in some villages) and second tremor, less than two hours later, registered between VI and VII. Eruption location undetermined because of heavy clouds.

July 1968 (CSLP 11-68)

New lava flow and crater explosions; 16 June activity was a tectonic earthquake, not an explosion

Information Report 2 (17 July 1968) "A new lava flow, accompanied by crater explosions every three seconds, is creeping down the slopes of Mount Etna, but there is no danger to villages on the mountain."

Information Report 3 (24 July 1968) The following report, clarifying the activities of Mt. Etna during the last months, has been received from Dr. Rittman. Seismic activity, reported to have taken place at Mt. Etna on 16 June 1968, actually was an earthquake of tectonic origin and was not related to Etna's activity.

"Since January 1968 Mt. Etna is in persistent activity with the usual fluctuations, due to oscillations of the dynamic equilibrium between the gas tension of the magma and the hydrostatic pressure of the magma column in th eopen vent. The NE-Crater ejects at intervals clots of glowing lava up to 250 m above the crater rim. Sporadically small flows of gas-poor lava issue from small sub-terminal boccas and may reach ont or two km of length. These more or less spectacular events can continue for months and years without reaching any paroxysmal climax.

"In contrast with such persistent activity, real eruptions are short lived violent events followed by a period of exhaustion... On 16th June 1968 there was no eruption at Mt. Etna... (but) a weak precursory shock was registered at 3 o'clock 54 minutes. A second, stronger one at 14h 10m. At 15 o'clock 3 minutes 34 seconds happened the third, very important shock (followed after two seconds by a similar one). The magnitude of this 3rd shock was M = 5.7 to 5.8. Epicenter: at 37.75°N Lat. 14.5°E Long. Hypocenter: at a depth between 50 and 60 kilometers. This earthquake is surely of tectonic origin and has nothing to do with the activity of Mt. Etna which lies more than 40 km to the E from the epicenter. Th islies on the southern slope of the Mts. Nebrodi, about 65 km from Catania and 100 km from Messina.

"Also Mt. Etna was shaken to such a degree that loose material on the inner slope of the crater have glided down into the vent, causing a cloud of dust which has been mistaken for an eruptive ash cloud."

July 1969 (CSLP 11-68)

Report of activity during a week of fieldwork at Bocca Nuova

Card 0656 (10 July 1969) Report of activity during a week of fieldwork at Bocca Nuova

Our mission of nine scientists and a 12-man support team, spent 7 days at the Bocca Nuova experimenting new devices to continuously measure and record the values of temperature, pressure and speed of the gases; chemical determinations, closely grouped (separated by the order of 10 seconds from one another), provided curves of the variations of the different constituents of the gas phase. Some figures can be given a indicative of the very rare phenomenon presented by this blow-hole: temperatures varied from ~800 to 1,100°C, speed from zero to ~20% in weight, CO2 content from 4 to 60%. The permanently eruptive state of Mt. Etna is presently characterized by: 1) lava fountaining in the NE crater with "bombs" hurled up to 250 m above the crater rim from any of the three vents opened in the bottom. 2) lava flows being poured out from a fissure opened on the W side of the NE cone; lava supplies vary up to 10 or 12 m3/s the flows do not exceed 1 km in length. 3) lava fountaining in the bottom of the deep (>400 m) main crater pit (so-called "la voragina"). 4) hot (>800°C) gas rhythmic activity at the June 1969 born blow-hole (so-called Bocca Nuova) on the west side of the cone containing "la voragina."

October 1969 (CSLP 11-68)

Since January 1967, Mt. Etna (figure 1) is in persistent activity. This activity can be observed if the vent of the volcano is open (the NE Crater of Mt. Etna) and the column of the fluid magma stands high in it. The gases escape through the crater producing intermittent explosions by which spatters and lumps of liquid lava are thrown in the ai sometimes as high as about 200 m. This explosive activity undergoes strong fluctuations in intensity. The intervals between consecutive explosions vary between a few seconds to many hours and even some days. It happens that during a long interval the scories lying on the inner slopes of the funnel glide down and cover the surface of the lava lake within the vent. For a while, only "smoke" issues quietly from the crater, till a new, somewhat stronger explosion throws the detritus our in the form of an "ash clouds" (crumbled scories but no real "ash"), and the usual persistent activity begins again, presenting often impressing spectacle, particularly at night.

It seems that some tourists observed the opening of a new "bocca" on the lower slopes of the NE Crater and believed that they were seeing the beginning of a new eruption, not knowing that during the normal persistent activity such ephemeral secondary boccas form very often – if the magma column stand high in the vent – and die out in a few days. In this case the degassing takes place at the surface of the magma column where trains of rising bubbles explode at intervals, throwing spatters out of the crater. The degassed epimagma descends along the wall of the vent while the foaming pyromagma rises in its central part. During the 2-phase-convection, the hydrostatic pressure may cause an intrusion of the epimagma between the strata of the more or less welded spatter and cinder cone of the NE Crater. At a certain distance form the vent, the increasing hydrostatic pressure in the downwards tended intruding epimagma becomes higher than the load and the resistence of the overlaying strata which are then uplifted giving way to the epimagma to the surface. A bocca forms without any explosive activity and the lava flows out quietly. Such a subterminal effusion is a common feature of the persistent activity of Mt. Etna. These small lava flows cool down rapidly and are covered by a slightly socriasceous glassy crust, under which the liquid lava continues to flow in a tunnel.

Fluctuations in the supply of epimagma, due to oscillations of the magma level in the vent, cause the formation of new lava channels and the ceasing of the former ones. In time, a great lava cake will form, covered by a consolidated crust. Here and there through this crust new secondary boccas are formed, from which ephemeral small lava flows are poured out. Such an activity may last for months and even for years without any paroxismal climax, i.e., without a true eruption, characterized by its violent character and by a following period of exhaustion with almost only fumarolic activity.

The only particular aspect of the actual activity of Mt. Etna is the existence of a large blow hole at the western part of the central crater from which, at short intervals, roaring gases escape violently. My friend and collaborator, Tazieff, with a team of specialists are studying this event carefully using new equipment which permits measurement of temperature, pressure and the speed and composition of the gases. All the qualities very widely within very short time intervals. The maximum temperature measured was 1070°C. During the night, large flames of burning gases are seen. Tazieff and his team will come to Catania within a few days to continue their research."

April 1971 (CSLP 33-71)

"A fissure opened a little above and halfway between the Volcano Observatory and the water condensation fumaroles. Lava flows, comparatively wide and swift, have almost reached the Observatory. Two points of emission and strong degassing craters on these fissures are delivering huge quantities of scoria and cinders with thunderous noise. The altitude of the activity is approximately 3,000 m. A party of six volcanologists will be there for field investigations on 7 April."

Card 1169 (08 April 1971) Activity from two radial fractures; three new cones built

Mount Etna thundered in new activity on 8 April 1971. Half a million persons watched to see whether it would threaten them. "For now it is not possible to determine whether the phenomenon will pose a threat to the countryside and the cities", said Swiss volcanologist A. Rittman, chief of the Volcanological Institute Center. The University of Catania Volcanological Institute said each of two radial fractures of the volcano's surface opened since 6 April near the 9,700-foot [(2,950-m)] level had 10 or 12 explosive centers hurling burning rock, ash, and steam into the air, and pushing molten lava down the mountainside. A report on 7 April said the more westerly of the two centers of activity already had thrown up three volcanic cones which were about 65 feet high. "It is the center of a violent uninterrupted explosive activity throwing hunks of lava and incandescent rock mor that 650 feet into the air accompanied by thunderous explosive noises." Lava flow from that eruption has surrounded three sides of the Volcanological Observatory, perched a half mile from Etna's main crater at 9,679 feet. The Etna cableway was halted because lava surrounded several of its supporting pillars. Etna's upper slopes are bare or snow-covered, but the rich soil of the lower elevations yields bountiful fruit and vegetable crops and has one of the highest population densities. The triangle between Catania, Nicolosi, and Acierale on Etna's southern flank supports more than 3,200 persons per square mile.

Two apparently radial fractures ~200 m apart; the W one 100 m long from 3,000 m elevation, the E one 100 m long from ~3,100 m elevation (figure 2). Presently two lava flows issuing from E fissure proceed ~3 km down W slope of Valle del Bove. One lava flow is presently flowing out lower end W fissure. It is fed by a small fountaining breached spattercone; its speed ~2 m/s, width 10-12 m, thickness up to our knees. One offshoot of western flow is leaning against north, east, and western walls of University of Catania Observatory. Other lava offshoots seriously threaten cableway. Length evening 8 April about 2 km with lava thickness 2-10 m, heights reached by observable molten lumps up to 300 m, speeds up to 200 m/s. Central crater big chasm and usually actively erupting; bocca nordest presently quiet and filled by whitish sulphur oxide smelling fumes.

Figure 2. Photograph of the eastern fracture and lava fountains at Etna, 7 April 1971. Courtesy of Haroun Tazieff. [Originally published in the CSLP 1971 Annual Report].

Card 1171 (12 April 1971) About 150,000 m2 were covered by noon on 6 April

"This is a report of the eruption between 15 April and 17 April. The two radial fractures are covered by lava flows. On the place of the E fracture, there is a single volcanic cone now 70 m high. There were violent explosions about every 20 minutes. At the foot of the active center there is a small cone delivering hot steam. Some ten meters below there is a vent with a lava flow which was going toward the Valle del Bove with a speed of 1 m/hour. This lava flow stopped Friday morning 16 April. The water condensation fumarole is preserved by the lava. The activity has momentarily stopped on the eastern sector fracture zone.

"On the place of the W fracture there are now two big cones, 40 m high on a N-S direction throwing incandescent rocks more than 200 m high. The southern one has on it flank a vent emitting 1 m3 of lava per second. This lava flow, after having entirely destroyed the observatory, has surrounded four pylons of the cableway, and the cable was broken by the lava Thursday morning, 15 April. The lava flow is still following its way at about 1 meter per hours. It is a typical block lava. Between Thursday evening, 15th and Saturday morning, 17th, the lava has moved about 50 m towards the intermediate station of the cableway which is now about 150 m from the lava. The northern cone stopped activity Thursday morning. According to the local guides it was during the night between 15 and 17 April that the explosive activity was the biggest since the beginning of the eruption. The observatory is surrounded on its four sides by four-meters-high lava products. The lava is threatening the north wall of the observatory and penetrated inside the observatory. The area covered by lava Saturday noon is about 1 square kilometer, and the damage is estimated to be about 1 million dollars."

Card 1183 (30 April 1971) Three new craters formed; lava flows advancing on the S flanks

"Etna activity which commenced 5 April continues. Three craters have formed S of the central crater. Two are currently active with frequency of 25-30 explosions per minute per crater, with ejecta being hurled up to 300 m. Lava flows were advancing to the SE (Valle del Bove - 3 km) and continue to advance to the S (cable car lines - 4 km), and SW (Frumento - 2 km).

"The first chemical analysis of lava indicates a phonolitic tephrite. Gas from active craters shows an excess of CO2 over SO2; gas from lava flows shows an excess of SO2 over CO2. N-S fractures have been observed on the E side of Monte Frumento and between Central Crater and Northeast Crater. Central Crater continues to smoke, mostly white, occasionally brownish-black. Northeast Crater shows no activity. Presently no danger exists for populated area."

Card 1184 (30 April 1971) Observatory surrounded, but not destroyed, by lava flows as of 25 April

The following corrects statements made on cards 1173 and 1174: "The observatory was not 'entirely destroyed' nor lava penetrated neither on April nor ... 25 April, when we last reached it (but to reach it, one has to cross the thick and still hot lava flows which do completely surround the building); the outpour of lava is an average of 25,000 m³ per hour presently emitted, at speeds varying from 0.5 to 4 m/s.; it is not a 'typical block-lava' but a quite typical aa flow with accessory pahoehoe streams."

Card 1185-1186 (30 April 1971) Two new explosive vents on the E fissure; cone growth continues

"The [E] fissure, which was the most active from the start, stopped all explosive and apparently effusive activity (although there are still some small and slow lava flows progressing on the big one which has flowed into the Valle del Bove. On the other hand, the [W] fissure extended upwards (northwards) for about 250 m with two new explosive vents starting on 19 April; they were still very active yesterday when we eventually reached them to collect gases (F. Le Guern). This fissure is now approximately 2.5 km long, extending en echelons from the very base of the central cone (alt. ca 3,050 m) down to the NW foot of Mt. Frumento Supino. Only its upper half km is active. The direction of this open fracture is N10-15°W. It is also the direction of the fault which worked last week (earthquake intensity 6-7 on the Mercalli scale) in the Giarre region, on the E foot of Mt. Etna. Exceptionally, some sort of relationship between an eruption and a tectonic earthquake could perhaps be considered as existing here.

"Presently, explosive degassing is continuously going on at the three lower and two upper main vents left open in the fissure. The lava lumps do not reach higher than about 200 m above crater rims. The cones are steadily growing up. The effusive activity decreased during the third week of the eruption and is more awkwardly estimated, the major part of the lava being now emitted through tunnels within the body of the main flow. This allows the lava keeping its heat content almost intact until its exit point, this meaning it being able to flow further down than the preceding ones in spite of decreased output and temperatures, as compared with the bigger flows of the first week of the eruption.

"R. Romano, assistant to A. Rittmann's I.I.V. magmatological laboratory, Catania, is carrying on daily analyses of emitted lavas. S. Cuccuzza-Silvestri, from the University of Catania, together with C. Sturiale and Riuscetti, also do watch the developments of the eruption. The two physicists of our eruptive gas-investigating group, P. Zettwoog and C. Vavasseur, will join F. Le Guern on 3 May. The violent degassing with related lava bombing, together with the strenuous access over fresh lava-flows, make it somewhat difficult to carry on P, T, and V measurements of the erupting gases as well as chemical analyses. The probes and protecting suits developed so far are suitable for the comparatively mild permanent eruptive activity of Mt. Etna (or Stromboli as well), but do not suffice for more violent eruptions."

May 1971 (CSLP 33-71)

Additional vents and fissures open; very active lava flows

Card 1195 (06 May 1971) Activity continuing in early May at high levels

"The activity of Etna has shown some increase during the past few days. A new fracture opened ... 4 May at the eastern base of the terminal cone between about 2,870 and 2,840 m above sea level and about 1 km from the crater's rim. Scoria cones were formed. An effusive bocca produced lava flowing with a velocity of about 20 m an hour at 1500 (local time) the 5th of May, along the W slopes off Valle del Bove. Explosive activity at the main focus of the eruption continues with somewhat reduced intensity, but the lava flows continue to be well supplied, the fresh material spewed through units on top of the old lava or sometimes running alongside. The lava is advancing in three main branches. The first south to southwest joined by the second moving south toward the central part of Piano del Lago about 2,880 m above sea level, while the third moving SSE flanks the main flow reaching down to 2,270 m above sea level by 1600 hours, travelling with an average velocity of 150 m an hour. Small lava flows running S or SSW continue to be emitted from the bocca above the principal eruptive focus at the higher level of 3,020 to 3,040 m above sea level."

Card 1198 (10 May 1971) Eruption stops at some locations, but opens new vents and emits more lava flows

"Early on the morning of 7 May activity suddenly ceased at the two eruptive centers above the former observatory, and at the four small bocche situated on a fissure to the E which had opened on 4 May. Towards the evening of 7 May, two new bocche opened to the NNE of the center of 4 May, one at a height of 2,700 m above sea level and the other a little to the N, and ca. 2,580 m. A little later, a third fissure with four orifices opened at ca. 2,540 m. All are emitting lava flows directed toward the Valle del Bove. In contrast to the earlier centers, the rather less explosive activity leads to the formation of hornitos rather than cinder cones."

Card 1200-1201 (13 May 1971) Details of lava-flow activity during early May

"Present volumetric output of lava was estimated this past week as follows: 5 May, ~140,000 m3/hour; 6 May, ~90,000 m3/hour; 9 and 10 May,3/hour. A new fracture system did open on 4 May about 2200 on the E foot of the central main cone of Mt. Etna along a trend perpendicular to the initial N-S one at an altitude of 2,890 m above sea level. High pressure gases without lava fountaining fumed at the upper part of the main new fracture. Intense fountaining, a big spatter cone approximately 30 m high about 100 m downslope, and a lava flow averaging 10 m in width and [a speed of about 9 km/hour] around was discovered on the down slope end of the fountaining crater. These eruptive fissures, perpendicularly opened during one [same] eruption and exactly one month apart, constitute a rather exceptional phenomenon. It is in fact really quite interesting. On 6 May former activity . . . located on the N-S fissure completely stopped and the big chasm in the central crater started emitting big ash-laden puffs. On 7, 8, and 9 May the new easterly trending fissure extended downward towards Pizzi Deneri across the Valle del Leone which is the upper part of the Valle del Bove.

"[Henceforward, explosive activity is concentrated in the new collapse-chasm formed in the E foot of Mt. etna's central cone, at the upper end of the fracture zone. Effusive activity] of 4-6 May stopped at the upper part of this fissure as similar although much lighter activity resumed at altitudes several hundreds of meters lower down.

"We were able to simultaneously make recorded measurements of speed by way of special speedometers, temperature by way of a thermocouple gage, and pressure by way of a special Pitot tube, and make a large series of gas samplings for chemical analyses which are being (analyzed) now. The temperatures were usually about 1,000°C and the speed (it is not yet exactly calculated) was about 400 km an hour. These gas samplings and measurements were made on maybe the best possible place just at the head of the erupting fissure where the degassing of the magma is optimum."

[Corrections of fissure locations by H. Tazieff, Card 1229].

Card 1202 (13 May 1971) Two additional eruptive vents open on 11-12 May

"Two new eruptive vents opened at an altitude of only 1,800 m above sea level S from [Rifugio] Cetelli. The first vent opened ... 11 May at 1137 exactly, and the second vent opened at 0400 [on] 12 May. These two vents have very slight explosive activity and the molten lava lumps are hurled only about 20 m high. The lava flows had already entirely crossed the forest zone and had reached the cultivated areas after having [crossed the road leading to Rifugio] Citelli. So, it's only an effusive . . . type of activity. All other activity stopped several days ago on the volcano, [but for the strong explosive degassing, without any fresh lava but with old ash- and rock-laden puffs at the new chasm, at an altitude of circa 3,000 m on the E foot of Etna's central cone.]

"The fissures system on the upper part of the volcano is always moving, and the fissures are extending, widening, and steam and hot gasses escape from an altitude of 2,670 m up to the very top of Etna at 3,300 m."

[Corrections of fissure locations by H. Tazieff, Card 1229].

Card 1208 (17 May 1971) Lava flow from 11-12 May centers cuts highway

"During the night 11-12 May two new effusive centers opened on a fissure directed NW-SE on the E slope of Mt. Etna at about 1,800 m elevation outside of Valle del Bove. The lower bocca is emitting a big lava flow which in 30 hours reached 1,100 m elevation cutting the highway Fornazza-Cabancitelli (Mare Niri). The effusive centers of 7 May on the inner slope of Valle del Bove are still very active. Up to now, no danger for inhabited places."

Card 1211 (18 May 1971) New fissure on the S flank emits lapilli and fumes

The following report was telephoned to the Center by H. Tazieff. "The fissure system with a trend N63°E is extending on about all the upper part of the E flank of Mt. Etna and at an altitude of 1,800 m above sea level it presently delivers two lava flows: the first one is approximately . . . . . . speed about 4 km/hour and an estimated output of 10,000 m3/hour; the second flow is approximately 8 m wide with a speed of 15 km/hour and an output of 10,000-15,000 m3/hour. Both flows coalesce lower down and proceed with decreasing speed down to 20-40 m/hour . . . . . the town of Formazzo which is still 5 km away from the lava front. Trees are burning and the menace becomes really important now.

"On the other hand, a new fissure opened today at an altitude of 3,000 m above sea level on the S flank just N from the new mountain hut called 'Il Torre del Filosofora' and this fissure starts throwing lapilli and fumes up into the air."

The following was telephoned to the Center by A. Rittman. "Emission of lava from the bocche near Cabancitelli is continuing and supplying copiously the great lava flow which advances slowly on a front about 700 m large towards the road Fornazzo-Lingualossa. Some farm houses and a vast area of cultivated land have been destroyed. In the early morning of 18 May strong explosions started to open a new sub-terminal crater which at 1000 was already more than 150 m wide and violently emitting dark clouds charged with ashes and white vapor. The new crater is situated at the place where on 4 May a small eruption center had opened on the E slope of Mt. Etna at about 2,900 m altitude. The activity of the bocche in the Valle del Bove continues much reduced; the Central Crater and the Northeast Crater quietly emit white vapor."

Card 1215 (21 May 1971) Summary of activity during 4-12 May

"A flank eruption has occurred on the E slope of Mount Etna, after 30 days of strong effusive and explosive activity located on the upper S zone of the volcano. The eruption started in the evening of 4 May by the opening of a new fissure at the E foot of the Central Crater about 2,900 m above sea level. The discharge of highly fluid lava increased during the night, and the strong and almost continuous degassing throwing lava lumps and incandescent scoriae 250 m began to build an elongated, sharp-shaped cone 30-35 m high. This cone was open toward the E, where a big lava flow was flowing with an initial speed of 10-15 km/hour. in a few hours the new lava reached a zone S of Monti Centenari in Valle del Bove near the 2,000-m level.

"This new activity apparently does not affect the S vents (figure 3), which remained active until 6 May. on this day, however, the explosive and effusive activity was somewhat decreasing. At midnight, the activity of the E vent began to decrease considerably and completely ceased on the 7th of May at 0200. In the morning of 7 May all was quiet on the volcano, excepting small cooling lava flows toward the S and some increased emission of gas from the Central Crater accompanied by rumblings and occasionally throwing brown cinder clouds.

Figure 3. Explosive craters at the south vents of Etna (elevations 3,000 and 3,100 m), at 1900 on 4 May 1971. Courtesy of J.C. Tanguy. [Not previously published.]

"By the 8th of May at 0100 we were able to observe a large red glow toward the E on the Valle del Bove, where a flank eruption was starting on the slope between Valle del Leone and Vale del Bove (level 2,700 to 2,400), near the 1950 eruption vents. Six or seven lava flows were vigorously active, but explosive activity was almost negligible. In the 9 May morning at 1100, our team reached the vents. There was a continuous cracking of ground between the now inactive upper E vent and the location of the flank eruption. This flank eruption had broken by two en echelon fissures, the upper being between 2,680 and 2,650 m, and the lower near 2,500 m. Several lava flows were moving at an initial speed of 1-2 m/s, while continuous degassing (but without violent explosions) was building small eruptive conelets (hornitos).

"In the morning of 12 May, the magma erupted at a more lower point by the aperture of a new vent near Rifugio Citelli, around 2,000 m level, and the lavas from this new outbreak could be dangerous for the inhabitants of the Etnean E zone. This flank eruption is of the 1928 eruption type, and is even located approximately on the same fracture line as the 1928 eruption was. The maximum lava temperature recorded by both thermocouple and optical pyrometers is between 1,100 and 1,130°C. The viscosity, as estimated by Jeffreys' formula, appears very low, from 1,000 to 10,000 poises only."

June 1971 (CSLP 33-71)

"Important developments are the engulfments which recently did enlarge both the central crater's big chasm (la Voragine) and the new eastern chasm, as well as the extent, in length, width, and throw, of the fracture zone which is responsible for the second phase of the present eruption."

[H. Tazieff also submitted a number of corrections to his previous reports and comments about other reported aspects of the eruption. The corrections have been made to cards 1185, 1200, 1201, and 1202, above. Additional comments regarding card 1195 are included below. Comments disputing the interpretation and information presented by A. Rittman in a 3 May 1971 report not published by SI, included on the original card, are not reproduced here.]

Regarding Card 1195 (06 May 1971): "The velocity of the lava flow escaping through the newly opened N63°E fracture was approximately 9 km/hour (see Card 1200) and the mentioned speed of 20 km/hour can only be that of some flow-front."

Information Contacts:H. Tazieff, Paris, France.

September 1971 (CSLP 33-71)

Activity starts again after 100 days of quiescence

Card 1294 (23 September 1971) Activity starts again after 100 days of quiescence

"[After] 100 days [of} complete quiescence, Mt. Etna volcanic activity started again. Lava column raised up to 200 m from Main Crater's rim. Two explosive boccas are located at the bottom of the crater."

December 1971 (CSLP 33-71)

"After a few months of complete quiescence -- last activity occurred around the end of September -- Mt. etna activity started again. Two explosive boccas are located at the bottom of the main summit crater named La Voragine. They emit periodically volcanic sands and ashes mixed with fragments of molten lava that reach about 400 m elevation falling into a surrounding area of about 300 m radius."

February 1974 (CSLP 20-74)

W-flank eruption rapidly builds cone; explosions and lava flows

Card 1791 (06 February 1974) First W-flank eruption of this century rapidly builds cone

"The eruption started [on 30 January] at an altitude of 1,800 m on the W flank of Mt. Etna. A new fissure was observed. The activity is essentially explosive. Only very short and thick tongues of lava seep slowly (less than 0.1-2 m/hour) to the E foot of a new rapidly built-up cinder cone about 50 m across the base and 100 m high. After initial, almost continuous lava fountaining which lasted throughout the first day, explosions occurred at the rate of 30-50 per minute. Lumps of a few kilograms were hurled 300-500 m high. Blocks weighing several tons have been observed reaching about 100 m above the crater rim. The lava temperature is estimated to be over 1,100°C in the crater and under 1,000°C in the lava flows. The viscosity is exceptionally high and probably due to a high degree of crystallization. The excepotionally high proportion of pyroxene accounts for the impoverishment of the lava in several magnesium elements and its relative enrichment in alumina and silica.

"Geophysical teams from the IIV in Catania, from the Observatorio Vesuviano, and from the Polytechnical School, Milano, started measurements on 4 February. It is totally impossible to reach the crater due to dense and continuous bombardment. I am presently sampling the eruptive gas phase. No eruption has occurred on the W flank of Mt. Etna this century. Local people called the quickly built-up new cone 'Mt. Rapido.' The latest report states that the 'explosions are stronger but still no lava flow.'"

Card 1795 (12 February 1974) Vigorous and frequent explosive activity from the new crater

The eruption that began on 30 January 1974 on the W slope of Mt. Etna was visited by J.C. Tanguy from 2 to 8 February.

According to Rittmann's classification, this should be an eccentric eruption with little or no relation to the main vent. It is mostly characterized by a highly explosive crater situated at an elevation of 1,680 m, which quickly built a cone over 100 m high. During the first days of activity, a restricted lava flow very slowly traveled about half a kilometer westward.

On 3 February, a very small lava tongue seemed to progress towards the SE, but next morning all effusive activity had stopped. The explosive crater, however, was vigorously active with continuous ejections (40-50 per minute) of big lava lumps and scoriae 400-600 m high. From time to time, the vent was blocked for a few seconds and the successive outbreak was accompanied by a strong air-shock percussion. Impressive also were the numerous earth tremors, especially E of the crater where the cone is partly open. Optical temperature at the vent is about 1,020-1,030°C, but measurements are probably too low as the top of the magma column cannot be observed. Another typical feature of this kind of eruption is the intense seismic activity the preceded the opening of the vent. Numerous earth tremors were felt from 14 January even at Rifugio Sapienza, 1,900 m high on the S flank of the volcano.

On 7 February the eruption was still continuing with some fluctuations. Explosions were 15-30 per minute, somwhat irregular in power and frequency. A new small but thick lava flow issued the day before from the N foot of the cone and is progressing as slowly as the former towards the NW. Earth tremors are persistent E of the crater.

Card 1807 (25 February 1974) Eruption ended on 17 February

"The eccentric eruption on the W slope of Mt. Etna ended on the 17th of February. On 8 February, the explosive activity began to decrease, although exceptionally strong recrudescences occurred from time to time. The outpouring of lava, however, was more important than at the beginning of the eruption. Nevertheless, it was still restricted in area and characterized by the sluggishness of the effusions. A flow starting on 9 February slowly reached a maximum length of about 2 km over the next few days. On the morning of 16 February, the explosions stopped at the vent and all outflow of lava ceased on 17 February.

"A run of lava temperature measurements was made using two chromel alumel thermocouples and a Schneider digital voltmeter. Although the eruption was coming to an end, temperatures as high as 1,075-1,085°C were recorded in both thermocouples. The optical surface temperature was 1,015-1,020°C. It can be compared to the measurements made at the explosive vent during the first phase of strong activity.

"The viscosity was high, but not measured (105 poises?), However, this high value can be partially understood with respect to superficial cooling connected with the slow rate of lava discharge."

March 1974 (CSLP 20-74)

New vent SW of the 1974 cinder cone opens on 10-11 March

Card 1814 (14 March 1974) New vent SW of the 1974 cinder cone opens on 10-11 March

"After 22 days of quiescence, eruptive activities started again on the W flank of Mt. Etna during the night of 10-11 March 1974. A new volcanic vent opened about 200 m SW from the cinder cone built up during the period of the earlier 1974 activity. The area is located about 8 km W of the summit crater. The vent is characterized by highly explosive activity which brought about the formation, during the first two days, of an 18-19-m-high cinder cone. A viscous lava tongue is flowing out [of] the vent very slowly, proceeding westward. The length of the lava flow was about 700 m after 14 hours from the opening of the vent. From an approximate examination of the erupted material the rate of emission during the first 14 hours approached 10-12 m3/s."

October 1974 (CSLP 20-74)

"The NE side of the terminal crater that stopped its activity at the beginning of the April 1971 eruption was opened again during the night of 28-29 September 1974 with strong explosive activity. During the night of 9-10 October 1974, following the collapse of the ring on the SW side, the lava lake that had formed in the base began to flow out. After 20 m, the flow changed its direction to the NNW for about 1 km. At the present time the activity is of irregular explosions and a slow flow of lava going for about 2,000 m."

Information Contacts:E. Lo Giudice and D. Condarelli, IIV.

February 1975 (CSLP 10-75)

An eruption began on the NW flank of Mt. Etna during the night of 23-24 February 1975, 24 hours after normal summit activity stopped. The eruption continues at the present time. Several spatter cones with fountaining activity have formed along the fissure which opened at an altitude of 2,500 m above sea level on the NW slope of Mt. Etna. A lava flow fed by the lowest vent has already reached a length of about 1 km. The front of this lava flow is presently about 10 km from the village of Maletto, to which it seems to be directed at the moment. The NE crater has stopped its activity, while the central crater is erupting dense clouds of ashes and sand.

Information Contacts:D. Condarelli, IIV.

March 1975 (CSLP 10-75)

New eruptive center on the North Rift zone

Card 2124 (19 March 1975) New eruptive center on the North Rift zone

On 24 February 1975, the persistent activity of the NE summit crater (altitude about 3,300 m) shifted towards the North Rift zone of Mt. Etna. Thus, a moderate outpouring of lava occurred at the 2,600-m level, accompanied by weak explosive activity that formed small so-called "hornitos." This new eruptive center was visited again on 13 March. The upper, sharp-shaped hornito (altitude 2,590 m) was continuously emitting white vapors and tiny pieces of scoriae only a few meters above the magma level. The lower hornito was inactive, being partly buried by early lava flows. The liquid lava itself poured out slowly 100-150 m downward from beneath a consolidated crust, forming several short-lived flows that did not exceed a few hundred meters in length. Emanations of gas were occurring also from a fissure near the 2,550-m level on the right edge of the lava field.

A maximum temperature of 1,065°C was recorded on chromel-alumel thermocouples inserted to 60 cm into the lava of the uppermost source. This temperature was nearly the same as those (1,075°C) measured in the lavas of the NE crater before 1971.

At the summit craters system there was on this day a strong continuous emission of dense white vapors from the NE crater and heavy clouds of brown ashes from the "Voragine" (chasm) of the central crater.

In mid-May, Etna [temporarily] ceased erupting lava. For several days prior to 15 May, the volcano's two lateral craters were inactive, and only a thin column of vapor rose from the central crater. A team of scientists, led by Haroun Tazieff, was on the scene conducting investigations.

Observations were made by the University College group and R. Romano of the IIV, 16 May-4 June 1976. Until this time weather conditions had been very bad on the volcano and reports of activity were sporadic. In the summit region, Bocca Nuova was fuming strongly and had a depth estimated to be in excess of 200 m. This pit was exploding throughout the period of observation, at rates varying from one explosion per 3 minutes up to 2 or 3 explosions/minute. Incandescent material was occasionally thrown above the level of the crater rim and vesicular scoria and Pele's hair fell close to the crater down-wind. The Chasm was also fuming strongly, and although its bottom was not seen, it had a depth of several hundred meters. During the afternoon of 25 May the Chasm started deep violent explosions at a rate of 20-25/minute. No bombs were seen from these explosions, and the activity died down over the next few days. The new pit on the W side of Northeast Crater was fuming quietly but not exploding. The area around this new pit contained large amounts of sublimates and its total depth was around 50 m. The Northeast Crater itself is now inactive, the vent being plugged by scree; but there is still heavy fumarolic activity high on the W inside wall.

The effusive activity during the period was occurring from new vents on the N side of the mountain (near Punta Lucia). A 40-m-high cone formed at about the 2,900 m level. Inside this cone was a conelet from which mild strombolian explosions of fresh gassy lavas were occurring on 26 May, though it was quiet on 31 May. Lava emissions were taking place farther downslope in the new lava field, from a number of boccas with positions that changed from day to day in an area above the 2,500 m contour. The rate of emission from one bocca was measured to be 0.4 m3/sec. and the total emission for the whole field was 2 m3/s.

Lava effusion and explosive activity, from a new vent at 3,200 m elevation just N of the Northeast Crater, began during the early morning of 16 July. A small lava lake, first observed at 0400, occupied the new vent, which was the source of two lava flows. The larger moved E into the Valle del Leone, and the smaller to the N. By 18 July, the flows had reached 2 km and 800 m length respectively. Strombolian activity began at 0515 on 16 July and rapidly increased in frequency and power. Ejecta reached 600 m in height and fell over an area ~1 km in diameter. The explosions had ended by 20 July and lava extrusion ended on 23 or 24 July.

Precision leveling two weeks before the eruption by John Guest and co-workers showed a 1-cm inflation of the S flank since September 1976, but strong deflation under aa fields on the N flank.

"After a period of quiescence since the beginning of this year, Etna erupted again on 16 July. New explosive and effusive boccas opened on the lower flank of the Northeast Crater cone at an altitude of 3,200-3,270 m. The first signs of eruption occurred in early July when a small pit opened on the N edge of the Northeast Crater Cone. Initially this pit emitted high-temperature gas which during the night was seen to be incandescent.

"The first sign of lava emission was seen by a guide at 0400 on 16 July when he observed a small 'lake' of lava in the bottom of the pit. An hour later weak explosive activity started and increased to become stable at about 15 explosions/minute by 1000. Incandescent material was thrown 300 m above the vent, with a maximum height of 500 m. The maximum range of the bombs was 400-500 m. During this early period of explosive activity, a fracture opened at about 3,240 m on the lower slopes of the cone and flows were emitted from the end of the fracture (at 3,220 m), first towards the NE and later to the E. At the same time ephemeral boccas (3,200 m) opened below the main fracture and fed a small flow in a northerly direction. Explosive activity reached its peak during the night of 17 July. During the early hours of 18 July collapse occurred in the Northeast Crater itself, causing great clouds of dust and ash. The explosive activity at the new vent then diminished both in frequency and intensity, becoming extreme1y variable.

"On 18 July the flows had reached a length of 800 m to the N and 2 km to the E. From then onwards the explosive activity continued to diminish, as did the effusion of lava to the N flow. The E flow reached the edge of Valle del Leone and continued as the principal flow, reaching a total length of 4 km by the end of the eruption. The N flow stopped with a total length of 1 km. Explosive activity ended at 1000 on 22 July and the lava flows stopped in the late afternoon.

"The area covered by lava was 0.16 km2, the volume of lava was 4.8 x 105 m3 and volume of pyroclastic material 500 m3. Microseismic activity was noted before and during the eruption.

"The Northeast Crater started erupting again on 5 August at 1430. The activity began with strong explosions from the July vent, with lava fountains being thrown to 400 m in the early stages. One lava flow formed, advancing N from a fracture on the N slope of the Northeast Crater Cone. The fracture was oriented approximately NNE. The lava was fluid and traveled about 3 km. Seismic activity was observed just before and during the eruption, which ended at 0630 on 6 August."

UPI reported that a third eruption began early 14 August and lasted only 14 hours. Fountains or strombolian ejecta rose 200 m above the vent, and two lava flows, each about 200 m wide, moved about 4 km down the volcano.

Etna's fourth eruption since mid-July began during the night of 2 November from the Northeast Crater Cone, site of the three previous events. Two lava flows were extruded, but (in contrast to the earlier events) explosive activity was very weak. The larger flow traveled 2.5 km NW from the vent and the smaller 1 km to the N, before the eruption ended about noon on 4 November.

Frequent eruptions from Etna's Northeast Crater continued through early January (table 1). Since the 2-4 November activity, progressively briefer 1-day eruptions occurred on 7, 22, and 25 November. On 6 December, extrusion of a single lava flow began at about 1100 from a NNE-trending fissure on the N side of the Northeast Crater. The flow traveled 4.5 km down the E flank before the eruption ended at 2200. On 24 December, tephra was thrown 1,000 m above the vent, and renewed activity 5 days later, accompanied by small earthquakes felt in nearby villages, projected tephra several hundred m above the vent. Lava extrusion resumed 2 January and the flow had advanced 1 km on a 150-m-wide front by evening. The eruption was continuing as of the morning of 4 January.

Etna erupted for the first time since January on 25 March [originally reported as 26 March], extruding a lava flow, destroying trees on the flank, and emitting ash, that was blown S towards Catania (25 km from the summit) by a strong wind.

An eruption began 29 April and continued as of 3 May. Lava was extruded from four new vents, on the SE flank, and had reached the 1700 m level near Monti Centenari, the 1852 cone by the morning of the 3rd. Explosive activity ejected bombs to 300 m above the largest of the vents (near the 1819 crater; figure 4) and built a 50-m cone. Explosions had declined by early 3 May, but vigorous lava effusion continued.

Frequent brief eruptions from Etna have occurred since July 1977, but all have originated from vents on the Northeast Crater cone. The last SE flank eruption occurred 5 April-12 June, 1971, destroying the Etna Observatory.

R. Romano reported the eruption started on 29 April at about 2000-2030 from bocca 1 (figure 5). Explosive activity built a cone that was 50 m high by 2 May. Lava flowed from the E side of the cone into the Valle del Bovee. Fissures opened, extending into the 1819 crater, where they intersected another fissure set which runs along the wall of the Valle del Bove.

Figure 5. Map showing locations of vents, fissure systems and lava flows of the April-May 1978 eruption. Bocca Nuova is W of the Chasm in the Central Crater; other boccas are identified by number. Prepared by J. Guest and J.B. Murray.

"During the afternoon of 1 May new boccas 2 and 3 opened on the existing fissures, then early the next afternoon bocca 4 opened, emitting a small lava flow that stopped the same day. Activity at bocca 1 also ended on 2 May, but the main flow front advanced at 100 m/hour. Bocca 3's activity diminished 6 May and ended by 7 May."

John Guest and J.B. Murray arrived at the volcano on 10 May and reported: "Lava effusion was limited to bocca 2, marked by a hornito 10 m high. The rate of eruption was 10 m3/s through midday 13 May when bomb ejection began at a rate that increased to 40-50 m3/s, but decreased to 20-30 m3/s by the next day and had returned to 10 m3/s by 15 May.

"Although bocca 1 was not emitting lava during this period, there were several collapses in the vent, producing billowing brown smoke. Occasional big explosions began 14 May, throwing bombs as much as 100 m above the vent. Explosions intensified on 27 May, but activity quickly returned to the 15 May level, and the eruption was continuing on 31 May.

"These eruptions have completely changed the Northeast Crater and surrounding area; the highest point on the volcano is now the Northeast Crater. Strong explosive activity during several eruptions has covered much of the summit area with ash, and during the Easter eruptions (March 25-28) there was a light ashfall as far away as Catania (25 km SE). Lava flows were extensive, one reaching as far down as 1,700 m elevation of the NW flank."

Etna's SE-flank eruption, which began on 29 April, stopped for about 12 hours on 26 May, then resumed and continued until the evening of 5 June. When visited by John Guest and others in late July, the main SE flank vents contained glowing red fissures and emitted jets of gas at high pressure. Occasional deep explosions could be heard inside Bocca Nuova (the W vent of the central crater), accompanied by rumbling and frequent collapse activity. The Chasm (the larger E vent of the central crater), normally continuously active, was largely filled with ash and snow, and showed no signs of activity. Steam emission continued from the Northeast Crater, site of a series of brief eruptions between July 1977 and March 1978.

Activity resumed on Etna's SE flank during the night of 24-25 August. The initial activity consisted of ejection of spatter bombs and ash from one of the 1971 eruption craters, at 3,000 m altitude on the SE flank of the summit cone. Lava extrusion from this crater began the night of 25-26 August and had ended by the next morning. Lava flowed eastward into the Valle del Bove, traveling 2.5 km to 2,000-2,100 m elevation. During the afternoon of the 26th, a second vent opened at 2,725 m altitude, on the wall of the Valle del Bove. Its flow moved 3 km in 12 hours, reaching 1,650 m altitude.

The explosive activity that started 24-25 August began to decrease on 27 August, but 7 more vents opened that afternoon on the walls of the Valle del Bove, between 2,800 m and 2,500 m. By 29 August, the number of active vents had decreased to four with a notable diminution of lava effusion, and explosions had ended.

The SE flank eruption ended on the morning of 30 August. Activity since then has been confined to occasional explosions from Bocca Nuova. Two distinct sets of fissures formed during the eruption. The active vents trended NE-SW, parallel to the 1971 vents. No lava was extruded from the second set, which trended N-S. Fault throws of up to 3 m were observed.

Further Reference. Mackey, M., and Scott, S.C., 1980, The eruption of Mt. Etna in August 1978, in Huntingdon, E.T., and others, eds., 1980, U.K. Research on Mt. Etna; Royal Society of London, p. 43-44.

A new SE flank eruption began on 18 November. Initial activity consisted of ejection of ash and wallrock from one of the spatterspatter cone cones formed in the August eruption. Ejection of incandescent ash and larger tephra from this cone started during the morning of 23 November, and was accompanied by minor lava effusion on the crater floor. Lava fountains rose 500 m above the August cone during the afternoon of 25 November and lava began to flow eastward into the Valle del Bove. That night, two new vents opened on the wall of the Valle del Bove at about 2,600 m altitude, extruding flows that traveled 4 km E, to an altitude of 1,500 m. Strong ash emission from the August cone was visible from 40 km away on 26 November.

Two more vents opened in the Valle del Bove on 27 November at 1700 m above sea level, and another vent opened nearby the next day, at 1,800 m altitude. Lava flows from these vents had traveled 4 km into the Valle Calanna, a steep valley extending SE from the E end of the Valle del Bove, by the evening of the 27th. A sixth vent opened that night at 1,650 m altitude, extruding lava that advanced slowly toward the town of Zafferana Etna (population 7,000), a few km S of Milo. The eruption ended during the night of 29-30 November.

By early July, activity had resumed at the Chasm in Etna's central crater. The Chasm had been continuously active for many years but had become quiescent and largely filled with ash during the 1978 eruptions. John Guest and coworkers arrived at the volcano 11 July and observed small, sharp, strombolian explosions from a small pit that had formed in the floor of the Chasm. When visited again on 27 July, the pit was filled with lava, covered by a thin crust that swelled prior to frequent Strombolian explosions. The lava lake had grown further by the next day. Large blisters formed in the lake, then burst, throwing bombs 200 m or more high. Some fell 50-60 m outside the rim of the Chasm.

Bocca Nuova, adjacent to the Chasm, was quiet on 11 July. However, collapse activity deep in this crater could be heard 27 July, and billowing clouds of dust were emitted.

Guest and coworkers observed the beginning of activity at a third site during the morning of 16 July, when strong gas emission started from a vent at the bottom of one of the 1978 craters on the upper SE flank (figure 5). Ejection of lithic blocks and a little fresh magma soon commenced, with the proportion of juvenile material increasing steadily. By afternoon, strong strombolian activity was occurring from the vent. The next day, bombs from many of the spasmodic explosions rose 200-300 m above the rim of the (100-m deep) 1978 crater. Most bombs fell back inside the crater, but a few landed as much as 50 m outside the rim. Another vent, on the side of the 1978 crater, emitted ash, building a small conecone. Similar activity continued until the night of 22-23 July, when the explosions became stronger and more frequent. The stronger activity continued through the morning of the 23rd, then declined to the more moderate levels of 17-22 July. Six vents were active at various times, two of which were dominant. This activity persisted, with some fluctuations in intensity, through 28 July, when Guest and coworkers left the volcano.

After a series of felt earthquakes 29-30 July, strong explosions from the Chasm began during the night of 3-4 August. Heavy ashfall took place in Catania, closing the airport, and ash fell as far away as Syracusa, about 80 km to the SSE. Unusual lightning accompanied the explosions, which were visible from the mainland, 75 km from Etna. Two fissures opened early 4 August near crater l, at 2950 m and 2875 m altitude. By afternoon, fluid lava from these fissures had traveled 13 km down the E flank, threatening the village of Fornazzo and forcing its evacuation. However, about 300 m from Fornazzo the lava changed direction, and damage was limited to about 1,000 acres of fruit and nut orchards. By late 4 August, summit explosions had apparently ceased.

Several new fissures opened the next day. The first was 1 km long, located at 1,800 m altitude in the Valle del Bove, on the SE flank. Others opened later on the NE flank, producing lava that flowed down two valleys. Lava effusion from some of these vents was continuing as of the morning of 7 August.

The following report was prepared by John Guest and Romolo Romano from observations by E. LoGiudice, D. Condarelli, and A. Pellegrino.

"In the afternoon of 3 August, lava fountaining up to a height of 300 m started in the 1978 crater, which we are now calling the Southeast Crater. This crater has been active since the middle of July. Ash from this explosive activity fell over the E flank of the volcano, then later that day fell over the SW flank as far as Syracusa, some 80 km away. During the evening of 3 August, an eruptive fissure opened near the 1819 Crater. Lava erupted that night reached Monte Centenari, some 2 km away. Fifty-four earthquakes were recorded with magnitudes up to 3.5-4.

"At 0545 on 4 August, another fissure opened in the Valle del Bove, SE of Monte Simone from 1,800 m to about 1,700 m elevation, 1 km long. Two flows were erupted quietly from the fissure. The flow from the top of the fissure moved SE (towards Rocca Musar-ra). The second, from the lower part of the fissure, traveled along the N wall of the Valle del Bove past Rocca Caora and reached the Torrente Fontanelle by midday. At 1430 the flow front was advancing at 100 m/hour and cut the Rifugio Citelli-Fornazzo road. The flow continued to advance, stopping in the evening 50 m from the N-S road through Fornazzo, just N of the town at 870 m above sea level.

"In the central crater area, large explosions had occurred from the Chasm during the end of July. At 1000 on 4 August, the magma level in the Chasm dropped, and in the Southeast Crater explosive activity was greatly diminished. At 1130 on 5 August there were again large explosions from the Southeast Crater and fountaining resumed at 1345 with heights of up to 400 m.

"At 1615 on 5 August, a new fissure opened just NW of the 1819 Crater, with fountaining. A lava flow from this fissure reached the foot of the wall of the Valle del Bove. At 1715, another eruptive fissure opened SE of the 1819 Crater, again with fountaining, and a flow moved into the Valle del Bove. Ash from this eruption also fell in the region of Catania and Syracusa. At 1730, a fissure with a NE trend opened at 2,500 m above sea level in the Valle del Leone. During the night of 5-6 August, lava from this fissure traveled 3 km.

"Early in the morning of 6 August, many earthquakes of up to M 3.5-4 were recorded until about 1218. In the afternoon, activity increased from the fissure near Monte Simone, which had been active on 4 August, and lava flows overlapped the earlier ones from this vent. The 6 August flow traveled some 1.5 km. At 2030, yet another fissure opened, with an ENE trend, at an altitude of 2,150 m, coinciding almost exactly with the 1928 fissure on the outer flank of the Valle del Bove. A sluggish flow followed the path of the 1928 lava, stopping on 8 August, 50 m from the Rifugio Citelli road, having traveled 1 km. Flows from vents in the Valle del Bove stopped on 9 August."

Activity resumed during the night of 1-2 September, with a collapse of part of the wall of Bocca Nuova and small explosions from the neighboring summit crater, the Chasm, the following day. The volcano then remained quiet until a 30-second explosion from Bocca Nuova killed nine persons and injured 23 at 1747 on 12 September. Some of the 150 tourists in the area at the time of the explosion were on the crater rim, and others were at a parking lot 400 m to the NW, where a large number of blocks about 25 cm in diameter fell. The explosion was apparently somewhat directed, because the distribution of blocks was dominantly to the NW of Bocca Nuova and no blocks traveled as far as 200 m in directions other than NW. No fresh magma was ejected by the explosion. The next day, considerable quantities of fine ash were emitted from Bocca Nuova and one or two small deep explosions were heard, but activity since then has been limited to weak emission of vapor containing SO2.

The following thermal anomaly report is from C. Archambault, J. Stoschek, and J. C. Tanguy.

"The existence of a N-S elongated thermal anomaly about 10 km has been determined on the upper half of Mount Etna, both by field measurements and infrared satellite imagery. It is believed that this high temperature anomaly is symptomatic of storage of magma at shallow depth within the rift zones of Etna. However, the magnitude of the thermal anomaly is expected to change with respect to volcanic activity (four flank eruptions have occurred from this zone during the past 2 years). Since 26 June, ground temperatures have been continuously recorded at the end of the thermal anomaly (figure 6). The results in this part of the volcano are considered the most significant because no volcanic activity has occurred in this zone for 70 years, therefore, the abnormal temperature cannot be due to cooling of residual magma. In order to eliminate the climatic effect, the data are presented (figure 7) as the difference between recordings of the central part of the thermal anomaly (Monte Calcarazzi Station) and those of a "reference station" (Sapienza) located at the same altitude about 1 km outside the anomalous zone.

Figure 6. Infrared image taken 16 September 1978 by the NOAA 5 weather satellite. The thermal anomaly is shown by the dark area around Monte Calcarazzi.

Figure 7. Graph showing the variation between temperatures recorded at Monte Calcarazzi and Sapienza, September 1978-9 August 1979.

"There had been considerable temperature increase (3°C) at the Monte Calcarazzi station during the month that preceded the flank eruption of 3 August. On 10 August the temperature at 120 cm depth, checked by direct measurements, was 9°C higher than the normal (Sapienza). Such a variation may be either a transient response to the August eruptive phenomena on the SE, E, and NE flanks, or the indication of magma motion southward - a problem that should be solved in the months to come.

[Some elements of this report were excised at the authors' request.] "Since the August flank eruption, no major volcanic event has occurred on Mount Etna, although on 12 September a moderate phreatic explosion resulted in nine casualties near the W vent Bocca Nuova. On 11 January 1980, ashes were emitted from both Bocca Nuova and the Southeast Crater, where activity has preceded all the flank eruptions for the past two years.

"However, ground temperature measurements made on 21 January in the S part of Etna (Sapienza, Monte Silvestri, Monte Calcarazzi) show that the thermal anomaly has been reduced to a very low level (1-2°C, in contrast to 9°C in August 1979). This is the lowest temperature recorded in this zone since temperature measurements were initiated in September 1978.

During the evening of 14 April, explosions began from Etna's summit area and red-hot gases were emitted from the Southeast Crater. Explosions continued for the next several days. Residents of Zafferana, 11 km SE of the summit, saw large explosions on 16 and 17 April that were especially spectacular at night because of incandescence or perhaps lightning. Poor weather prevented observations from the IIV in Catania.

Etna guides who climbed the volcano, probably on 17 April, saw large fresh bombs near the Chasm and Bocca Nuova craters. Bombs were particularly prominent on the N and W sides of the central crater area. On 27 April at 1705, a large summit explosion produced a 3-km-high ash cloud. Renewed explosions began during the afternoon of 29 April and continued through the night from the Southeast Crater.

"After a 3-month period of stabilization, ground temperatures in Etna's S-flank fissure zones (6 km S of the summit) began to increase again in March (figure 8). However, station 3, only 25 m from station 2, continued to show a nearly constant temperature. Variations in temperature are calculated by comparison with Sapienza reference station 1, located outside the fissure zones, 0.5 km WNW of stations 2 and 3, and 1.25 km from station 5.

"The increase in temperature preceded renewed activity from the summit crater system. On 14 April, red-hot gases were emitted from the Southeast Crater and on the following days fresh lava lumps were ejected from the Chasm.

"On 20 April, S flank temperatures were stationary or even slightly lower. Between 20 April and 1 May, temperatures strongly increased again. A large explosion on 27 April (probably at Bocca Nuova) and strong lava fountaining on 29 April at the Southeast Crater were followed by moderate magmatic activity until at least 3 May. After a short period of stabilization (1-4 May), temperatures were still increasing, although more slowly.

[Archambault, Stoschek, and Tanguy added the following paragraph to replace explanatory material excised from 5:1]. "It was initially believed that the thermal anomaly fluctuations were related to the reopening of cracks caused by increases in volcanic pressure. Further investigations showed this hypothesis to be incorrect, with the systematic seasonal increase of surficial temperatures being mainly the consequence of a microclimate (Bourlet and Bourlet, 1982). A volcanic effect does occur, but its influence cannot be simple and is probably linked to the circulation of hot waters through the S flank. From this standpoint, a striking example is the rapid temperature increase measured in November 1982 at 6 m depth (figure 8, 08:04), where a climatic effect cannot be invoked. Such a variation was not observed the following years (November 1983 and 1984). It may have resulted from the heating of meteoric waters by an intrusion of magma into the S flank a few months before the March 1983 eruption.

"The Southeast Crater was intermittently active throughout May. On 31 May, two vents were observed inside the crater with moderate ejections of incandescent magmatic material up to 50 or 100 m, with some bursts occasionally reaching 200 m. As of 3 June, activity had increased noticeably, and explosions were stronger and more frequent. The number of explosions per hour reached 165 and the ejecta reached heights of 200-300 m."

Activity increased gradually between 13 and 20 July in the Southeast Crater, site of lava fountaining in late April, intermittent eruptions in May, and stronger explosions in early June. No information was available on activity between 4 June and 12 July [but explosive activity was persistent from the SE Crater and Bocca Nuova]. In mid-July, many small explosions were observed on some days, while other days were characterized by fewer but larger explosions. After a period of poor weather, renewed observations of the Southeast Crater on 31 July revealed four active vents, located 25 m below the lowest portion of the crater rim. The two larger vents steamed continuously, and exploded about every 2 seconds, ejecting incandescent tephra to heights of 25 m. At approximately 2-minute intervals, stronger explosions sent tephra upward 100 m. A small spatter cone surrounded the vents. The two smaller vents ejected juvenile material only occasionally.

Explosions from deep within Bocca Nuova were heard about every 2 seconds on 31 July. No tephra reached the rim. Since last September, Bocca Nuova's diameter [corrected from radius] has increased by as much as 80 m, bisecting a small crater formed in 1964. Most of the increase occurred during the winter, but further crater growth took place in June.

The Chasm, E of Bocca Nuova, was filled with solidified lava to within 25 m of its rim on 31 July. Large amounts of spatter and many bombs surrounded the crater as a result of activity in April (05:04).

Gravimeter readings made in late July by Tim Sanderson on the S flank yielded values that were significantly lower than in September 1979, indicating a loss of mass on that flank. July 1980 N flank gravity values were very similar to those of the previous September.

Relatively weak activity similar to that of late July continued through August at Etna. A 1-day eruption on 1 September deposited ash on the E flank and extruded two lava flows.

John Guest climbed the volcano on 18 August. As on 31 July, explosions occurred deep within Bocca Nuova. The Chasm remained quiet. Mild strombolian activity continued from the Southeast Crater, but reportedly weakened the following week. Romolo Romano reported that a swarm of local seismic events began on 21 August.

At 0957 on 1 September, a pale brown plume was seen rising from the Northeast Crater, which last erupted in March 1978 (03:05). By 1130, explosions were ejecting large bombs or blocks every 10-15 minutes. An increase in seismicity at about 1700 was followed at 1730 by stronger explosions that were audible in Fornazzo, 10 km E of the crater. A large black eruption column rose to 6 km above the crater. By 1800, ash was falling on Fornazzo and the entire E flank. Geologists reached the eruption area by about 2000 and saw nearly continuous strombolian explosions from two vents in the Northeast Crater ejecting tephra to 500-600 m above the rim. Lava from the Northeast Crater flowed to the N and NW. By the next morning, the eruption had stopped. Heavy fog made mapping of the two lava flows difficult, but the NW flow had moved past Punta Lucia, about 3/4 km from the Northeast Crater.

A second brief eruption from the Northeast Crater began early 6 September, ending at about 1500 the same day, after an estimated 10 hours of activity. A small lava flow was extruded. No further information was available at press time.

The 6 September eruption began at about 0500. Vigorous strombolian activity continued for 10 hours, ejecting bombs to 500 m above the same Northeast Crater vent that had erupted 1 September. Bombs 20 cm in diameter or larger fell as far as 750 m away. A lava flow, extruded from the same vent at a rate of 10-20 m3/s, traveled 2 km to the N, directly over the main lobe of the bifurcated 1 September flow, which extended about 1/2 km farther to the N. Ash fell on the coastal towns of Acireale and Taormina, 20 km SE and 30 km NE of Etna. Observations after the eruption showed the vent completely filled by lava and slumped debris. As of 3 October, no further eruptions had been reported.

Rumbling and deep explosions continued in September from Bocca Nuova. The Chasm remained inactive. Mild strombolian activity at the Southeast Crater stopped in September, with only infrequent gas emission and small collapse events reported.

Tim Sanderson collected gravity data before the 1 September eruption and after the 6 September one. Frequent ground temperature measurements by J. C. Tanguy and associates continue (05:05).

The IIV reported explosions and extrusion of lava from the Northeast Crater. After a period of ash emission at the end of January and the beginning of February, stronger activity began with intense explosions the evening of 5 February. Lava flowed through a breach in the W-to-NW side of the Northeast Crater cone, forming three lobes that moved W, NW and N, covering the upper NW slope of the volcano. The N lobe, the largest, traveled ~2 km to 2,600 m elevation where it had a 1.2 km front. The eruptive activity stopped the evening of 7 February.

An eruption 17-23 March extruded lava from several fissures on the NNW flank. Initial estimates indicate that the main flow reached 7.5 km in length, lava flows covered an area of 6 km2, and 30-35 x 106 m3 of lava were extruded [but later calculations from topographic measurements by Murray (1982) yield 18 x 106 m³] at a rate of 58-70 m3/s. Damage was estimated at about $10 million. Of the 90 historic eruptions of Etna for which location data are available, only three (1614, 1764, and 1918) occurred on the NW or NNW flanks.

Etna began to erupt on 17 March after a 2-day swarm of about 500 earthquakes, including a M 4-5 event during the morning of 16 March. On 17 March at 1337 an eruption fissure opened at about 2,550 m [corrected from 2,250 m] above sea level on the NW flank, trending approximately NW-SE. Lava fountains rose 100-200 m from this fissure and lava flowed rapidly westward. In the next 4 hours, three more fissures opened, the first and third also trending NW, the second WNW. All showed strong lava fountaining and were the source of lava flows. As fissures formed at lower altitudes, those higher on the volcano ceased to be active.

At 1855 on 17 March, another fissure opened at 1800 m elevation on the NNW flank, trending NW at its upper end, but after a short distance changed direction to more directly downslope. A large lava flow that originated from this fissure traveled 5 km within 4 hours, cut a railroad and highway (at 730 m altitude) during the night, and crossed another railway line and road (at 680 m altitude) early on 18 March. The lava destroyed orchards and farm buildings, and passed very close to the village of Montelaguardia, forcing the evacuation of its 250 residents. The fissure propagated downslope to 1,300 m altitude at 1130 on 18 March. The lower section extruded a small lava flow that briefly threatened Randazzo (population 15,000) but did not force its evacuation. By 1630, the center of the main flow was more than 1 km wide and its front had reached 650 m altitude, 100 m from the bed of the Alcantara River.

At 2200, another fissure opened between 1,235 and 1,140 m altitude extruding flows that moved toward Randazzo. By this time, the system of eruptive fissures had a total length of about 7.5 km. The main flow reached the Alcantara River bed (600 m above sea level) on 19 March at 1100, while the flows extruded from the fissure between 1,235 and 1,140 m altitude continued to advance slowly. By noon on 20 March, this fissure was characterized by mild spatter ejection that continued to feed slow-moving lava flows. However, the main flow had nearly halted. Sporadic activity between 1235 and 1140 m continued 21-22 March, finally ending during the evening of the 23rd. The longest flow from this fissure stopped at 900 m elevation, 2 km from Randazzo. More than 25 small earthquakes were recorded on 23 March, centered around the eruption fissures.

Throughout the period of lava extrusion, more or less intense emission of sand-size tephra occurred from Bocca Nuova, enlarging it to the W. Strong winds caused flank ashfalls on 22 March [as the Northeast Crater briefly ejected juvenile cinders; Tanguy and Patané, 1984].

April 1981 (GV11 81:04)

Photograph from Space Shuttle

The fissures and lava flows on the NNW flank resulting from the March 1981 activity (figure 9) were visible to astronauts aboard the Space Shuttle in April 1981.

Figure 9. Photograph of Mt. Etna and vicinity taken from the Space Shuttle in April 1981. North is towards the top of the photo; Catania is SE of the volcano on the eastern coast of Sicily. Photo no. STS 1-13-444, courtesy of Charles A. Wood.

On 29 July, a dense ash cloud was ejected for more than 30 minutes from Bocca Nuova. The ash emission may have been produced by collapse within the crater; no significant explosions were associated with the activity. Similar events have occurred on several occasions since the March eruption.

Collapse activity deep within Bocca Nuova has been frequent since the 17-23 March fissure eruption. No fissuring or other evidence of surface collapse has been observed around Bocca Nuova. Explosions associated with the collapse activity ejected fine ash, caused strong ground vibrations 300 m from the crater, and could be heard as much as 10 km away. Plumes produced by this activity could sometimes be seen on the satellite images returned once daily by the NOAA 7 polar orbiter. Images returned shortly after noon on 3 and 4 October showed narrow, well-defined plumes extending ~75 km downwind from Etna. A smaller, less dense plume extending outward only about 20 km was present on the 6 October image.

Images returned by the NOAA 7 polar orbiting satellite continued to show occasional plume emission: on 9 October at 1442 plume roughly 75 km long, drifting to the SE, and on 1 November at 1329 a much smaller plume, roughly 20 km long was moving to the SW. No other activity was seen on the almost daily imagery between those dates.

As of mid-January, collapse on Bocca Nuova's inner walls was almost continuous, producing plumes that contained fine ash but no fresh magma. At times of little or no wind, the plumes rose 3-4 km above the crater. There was no evidence of collapse beyond the crater rim. No changes have occurred in seismicity or tilt.

Images from the NOAA 7 satellite showed plumes emerging from the summit area on 4 and 6 January. A plume observed on 4 January at 1431 extended ~80 km to the SE, beyond the coast. Infrared data showed that the plume's apparent temperature was comparable to that of the sea water beneath it, and thus it probably remained at a relatively low altitude. The next image of the area, at 1408 on 6 January, showed a fairly diffuse linear plume ~55 km long, drifting SE. A plume from Etna had last been observed on satellite imagery (available most days for the Etna area) on 1 November.

[Some elements of this report were excised at the authors' request.] Since the fissure eruption of 17-23 March 1981, explosions associated with collapse activity deep within Bocca Nuova have produced small to moderate ash plumes. Magma was observed in the bottom of Bocca Nuova in mid-May, mid-July, and early September 1981, and incandescence was seen there in February 1982. All activity from Bocca Nuova stopped 8 May, but resumed about a week later at around a time of unusual seismic and thermal activity on the NE flank. Incandescent scoria rose above the crater rim 20-22 May.

Seismicity increased during the third week of May and culminated with an explosion, possibly at 1515 on 27 May, when the summit seismometer operated by the Univ of Catania detected a M 3.5 earthquake that was felt by local residents. The next day, geologists found blocks of old lava that had been ejected more than 300 m from the Chasm. Blocks up to 1.5 m across were found in small impact craters at the rim. Many blocks larger than 10 cm occupied elongated depressions, implying relatively oblique impact, oriented radially to the crater. The greatest concentration of blocks was immediately N of the crater, although the pattern of smaller blocks suggested a NW orientation. The depth of the Chasm had increased from ~50 m in March to ~100 m after the explosion, by removal of material that had filled it since 26 May 1980 [explosive activity had begun in mid April], and vents in its walls were steaming. However, activity 28 May was concentrated in Bocca Nuova, where occasional detonations could be heard and the crater floor was obscured by sulfurous steam. On 29 May, Bocca Nuova ejected black ash, containing fresh magma, every 2-3 minutes. Ash clouds rose 200-300 m above the crater rim and were blown a few hundred m to the south. Only fumarolic activity was observed in the Northeast and Southeast craters.

Etna erupted on 8 August, when a black tephra column was emitted from Bocca Nuova and lava rose to 150-200 m below the crater rim. The activity was accompanied by a marked increase in minor earth tremors. Sicilian authorities have restricted tourist access to the volcano.

Strombolian activity on the floor of Bocca Nuova was observed in September. Fine ash fell on the crater rim. Since summer 1981, continuing collapse activity at Bocca Nuova has widened the crater by nearly 100 m, to 250 m in SW-NE dimension. Poor visibility prevented determination of the crater's depth. Just to the E, vapor emission was continuous from the Chasm, site of an explosion in late May that removed ~50 m of debris that had choked this vent for 2 years (07:05).

A C-141 cargo plane pilot observed tephra from Etna at ~12 km altitude on 6 December at 1800. Romolo Romano reported there had been many small explosions from Bocca Nuova throughout the day. No eruption plume was visible on the only satellite image of Etna that was available on 6 December, from a NOAA polar orbiter at 1530. Poor weather and heavy snow make access to Etna's summit area difficult during the winter.

Explosive activity of varying intensity continued through early January from the floor of Bocca Nuova. Large ash emissions were sometimes observed. During the night of 24-25 December, intense explosions ejected incandescent tephra. Most of the tephra fell back within the crater, but some was deposited outside the crater rim.

Press sources reported emission of large quantities of gray and white "smoke" from the Northeast Crater but Romolo Romano noted that the Northeast Crater activity was fumarolic and no ash was ejected. The Northeast Crater last erupted in February 1981, producing a lava flow and ash (06:02).

A destructive S-flank fissure eruption began on 28 March, preceded by a series of strong earthquakes first felt during the night of 26-27 March. At about noon on the 27th, a strong smell of H2S was noted from an old cone (Monte Silvestri) roughly 2 km S of the initial eruption, although H2S is not normally present in that area. Seismicity continued through the following night. At about 0845 on 28 March a NNE-SSW-trending eruptive fissure opened from about 2,450 to 2,250 m altitude, roughly 4 km S (bearing ~170°) of the central crater (between the eruption fissure of 1910 and La Montagnola). The base and E side of this fissure fed several lava flows that initially moved to the SSE and SSW then turned S. Weak explosive activity along the entire fissure ejected modest quantities of lava fragments. By evening, the main flow had cut a road and overrun several buildings.

During the morning of 1 April, vigorous emission of gas, ash, and old lava, accompanied by occasional phreatic explosions, began from two explosion craters upslope at 2,700 m altitude. At the end of the day, explosions from the southern vent ejected lava fragments. On 2 April, nearly constant lava production fed numerous superposed flows that formed a 500-m-wide lava field extending to 1,900 m altitude. As of 3 April, the lava had not advanced below 1,450 m altitude, 3.5 km from the fissure. At least four principal effusive vents were active along the 750-m fissure, and from its upper part strong gas emission with sporadic explosions occurred at about 30 hornitos.

Bands of open fractures, oriented about N-S, extended from the central crater area to the eruptive fissure. A substantial widening was noted at the S rim of Bocca Nuova, site of frequent collapse activity since Etna's last eruption (from N flank fissures in March 1981). Strong vapor emissions from Bocca Nuova sometimes included abundant ash. There was no activity from the Northeast and Southeast craters.

The temperature of the lava was less than 1,100°C and its chemistry (alkali basalt) [corrected from phonolitic tephrite] was similar to that from some of the more recent eruptions. An area of more than 1 km2 was covered by lava and the volume emitted was estimated at about 8-10 x 106 m3. The IIV considered the eruption to be a typical slow subterminal type. The last activity of this type on the S flank was in 1780. As of 8 April, effusive activity had diminished, but the eruption had not yet ended.

The lava destroyed ski lifts [the cable car system originally reported destroyed survived until March 1985] and destroyed or seriously damaged nine privately-owned huts and 11 small buildings owned by local authorities, including restaurants, chalets, mountain refuges, and a first aid station. Lava remained 8 km from the village of Nicolosi, its closest approach to a village or town.

The eruption was continuing as of 6 May. J. C. Tanguy noted that temperature station cables in the S part of the summit zone broke on 27 March between 0530 and 0946, probably because of the opening of the eruption fracture. Fissures extended S from 2,700 m (where two explosion vents formed on 31 March) to 2,450 m, then turned SSW along the trend of the 1910 eruption. Small hornitos and spatter cones formed between 2,450 and 2,350 m. The main effusive vent was at 2,280 m altitude. On 31 March, the temperature of lava at this vent was 1,067°C at 60 cm depth, and the same value was measured in the main flow 30-40 m downslope on 4 April. F. Mousnier-Lompre and G. Scarpinati recorded a lava temperature of 1,078°C on 1 May. J.B. Murray and A. Pullen reported that lava flow surface velocities measured 17-30 April were in the 1.17-3.41 m/s range. Using surface velocities, Murray and Pullen calculated an average effusion rate of 22 m3/s if flow thickness was 3 m and 44 m3/s if the thickness was 5 m.

Romolo Romano reported that lava flowed S and SW, forming a wide, complex field as much as 1 km across and 40 m thick. On the E side of the field, the fronts of the longest flows were about 6 km from the vent and reached 1,150 m altitude before stopping. On 23 April, the principal flow shifted to the W side of the field, and advanced to about 6.5 km from the vent, stopping at 1,100 m altitude only 30 m from a road. On 4 May the primary flow was again moving down the E side of the lava field and had reached 1,450 m altitude as of early 6 May. Lava flowed from the vent at about 2 m/s, a rate of about 10 m3/s, and had a temperature of about 1,030°C [but note 1,067° and 1,078° above]. The rate of advance of the flow fronts was quite variable and dependent on the gradient; estimated velocities ranged from a very few meters per hour to ~60 m/hour. The area covered by the lava was about 4 km2 on 6 May and the volume of lava emitted was ~40 x 106 m3.

Earlier in the eruption, small hornitos and spatter cones had formed above the main effusive vent and small quantities of lava fragments were ejected. Vapor emission continued from this portion of the eruption fissure but no lava fragment ejection has been noted since 10 April. Two explosion vents [not one as originally reported] at 2,700 m altitude ejected ash during the morning of 1 April [not 28 March as originally reported; Tanguy and Patané (1984), Frazzetta and Romano (1984)], but only vapor emission has been observed at this vent since then.

Throughout the eruption, ash was ejected at varying intensity from Bocca Nuova. More vigorous ash ejection 2-5 May sent plumes to 2 km above the crater and caused ashfalls on the mainland (Calabria coast) at least 60 km from the volcano. On 4 May, a polar orbiting satellite image showed the beginning of an ash emission episode at 0453. By the next image at 0606, a narrow plume extended nearly 1,000 km SE from Etna at an altitude of about 7.5 km. Venting was continuing at 0945 but the plume was smaller; at 1603, it was 100-150 km long. Another image, at 0705 on 6 May, showed a plume ~100 km long. From the Chasm, weak emission of gas that sometimes contained reddish ash has been observed during the eruption. In the Northeast and Southeast craters, eruptive activity was limited to weak fumarolic emissions, but new concentrations of large fractures were seen in these craters as well as small internal collapses.

Murray and Pullen reported that vertical ground deformation during 1981-82 was characterized by summit deflation and S flank inflation. Reoccupation of a precise leveling network 20-22 April 1983 showed large changes on the upper S flank (near Piccolo Rifugio) since September 1982: 42 cm of uplift W of the eruption fissure, 12 cm of uplift E of the fissure, and a drop of 126 cm on the fissure itself. Large and complex movements had occurred elsewhere, including deflation of 76 cm above the eruption fissure at about 2,900 m altitude (near Torre del Filosofo). Horizontal measurements across the fissure showed an E-W extension of 1.3 to 2 m since August 1981, with contraction of 17 cm and 6 cm at 500 m and 1 km W of the fissure. A network of 25 dry-tilt stations occupied 24-29 April 1983 showed no general inflation or deflation of the edifice since 1982.

Ground temperature measurements at 6 m depth on the S flank at 1,900 m altitude showed a strong increase in November 1982 (figure 10). In the summit zone, ground self-potential increased at roughly the same time and again in mid-January. A very steep rise occurred in early April, after the start of the eruption (figure 11). Immediately before the eruption a geophone in the S summit zone registered strong seismic activity, up to 134 events/hour during the morning of 27 March.

Figure 11. Ground self-potential (top), and temperature (bottom), at 120 cm depth, recorded at Vulcarolo station, south summit zone, 2,965 m altitude. Note that the very steep rise in self-potential occurred in early April, after the start of the eruption. Telemeasurements by ARGOS system.

The continuing eruption showed signs of a progressive decline on some days. The velocity of the lava flow from the vent at about 2,280 m altitude decreased from ~1.7 m/s on 28 May to <0.5 m/s in early June, with a reduction in the rate of outflow to ~3 m3/s. In early May, the velocity at the vent had been about 2 m/s and the effusion rate was about 10 m3/s. New flows, most of which moved S and SW, continued to add to the S flank lava field that has accumulated during the eruption. On 25 May a flow advanced beyond the edge of the lava field, reaching 1,240 m altitude by the 27th, ~5.5 km from the vent. From 28 May, all of the lava flowed toward the interior of the lava field or approached its W side. In early June, frequent and impressive overflows occurred from both sides of the main lava channel at about 2,200 m altitude. Within the lava field, effusive pseudo-vents at around 1,800-1,980 m altitude have remained numerous. As of 8 June the area covered by lava was estimated at 6 km2 and the volume of lava produced by the eruption was ~75-80 x 106 m3.

Explosions of varying intensity continued, particularly from Bocca Nuova and primarily ejected reddish ash. On 15 May and 1-4 June the explosions were quite large. A NOAA 7 satellite image at 1529 on 15 May showed a strong plume (as large or larger than the 4 May plume; 8:04) that extended 100-150 km to the SE. Pilots reported that the plume had emerged from the summit area at about 1100. on 1 June at 0800, pilots estimated the height of a plume at about 5 km and reported that it was drifting SSE. In May and early June, only vapor emission was observed from an explosion crater at 2,700 m altitude and a fissure between 2,450 and 2,300 m altitude that were active early in the eruption. On 3 June, deep felt shocks (M 3.3) occurred near the active vent; these events continued the next day.

Because the advancing lava flows threatened additional property damage, efforts were made to divert the lava. Explosive charges were detonated at 0409 on 14 May to blast a passage from the W side of the main lava flow at 2,100 m altitude into a previously prepared artificial channel. Initially 20% of the total lava flux was diverted into the artificial channel, but by 16 May lava had ceased to flow in this channel after reaching 700 m length. After the explosions, however, there were numerous substantial overflows from the main natural channel, particularly from the W side, and these slowed the advance of the most distant flow. Artificial embankments have also been constructed E and W of the main natural channel, allowing control of the overflows from this channel that were moving over earlier flows.

Pham Van Ngoc and D. Boyer obtained self potential data during a PIRPSEV mission to Etna 27 April to 3 May. The following is a report from Pham Van Ngoc.

"Five self potential (SP) profiles were carried out from 2,350 to 2,700 m elevation. These profiles straddled the open fissure that trends NNE-SSW below Piccolo Rifugio (at 2,500 m altitude on the S flank) and the N-S fractures above it. The profiles were located upstream from the lava emerging at 2,320 m altitude. Figure 12 shows the results of the SP profiles.

Figure 12. Results of self potential profiles obtained 27 April-3 May across open fractures that trended N-S above 2,500 m altitude (Piccolo Rifugio) and NNE-SSW (about 200° azimuth) below 2,500 m. The topographic profile (vertical exaggeration 2:01) parallels the fissure trends, changing strike at Piccolo Rifugio. Arrows pointing left show positions of self-potential profiles. Arrows pointing right diagrammatically show the inferred subsurface flow of magma, its emergence from the vent at 2,320 m altitude, and continuation downslope as flowing lava. The inset at lower left details profile III, indicating that the axis of the subsurface flow was under the ski lift building, 40 m W of the open fissure. Here arrows show local stress field and fault movement. Courtesy of Pham Van Ngoc.

"The shape of the SP anomalies was very different S and N of Piccolo Rifugio. To the S, profiles I, II, and III show huge (more than 50 mV) and sharp anomalies suggestive of superficial origin. it is notable that the maxima of these anomalies were not located just above the open fissure, but 40-50 m westward. The detail of profile III, just below Piccolo Rifugio, indicated clearly that the axis of the underground flow lies 40 m W of the active fissure (under the ski lift building W of Piccolo Rifugio). These results confirm that the pressure exerted by underground flow induced an E-W extension, creating a small graben clearly visible in the area of Piccolo Rifugio. The open fissure corresponds to a normal fault on the E edge of the graben and caused the collapse of the W part of Piccolo Rifugio.

"Above Piccolo Rifugio, profiles IV and V show much smaller Sp anomalies (~250 mV). Furthermore, these anomalies widen, thus indicating a deeper origin. It is notable that the shape of the SP anomalies changed completely from profile III to profile IV in a distance of ~300 m.

"SP results suggest that: 1) above Piccolo Rifugio, the magma flowed deeply in a N-S direction; and 2) level with Piccolo Rifugio (2,500 m altitude), the magma suddenly approached the surface and followed a shallow underground NNE-SSW channel that ran some 50 m W of the open fissure, then came to the surface at about 2,320 m altitude. Arrows in figure 12 indicate the path inferred from the lava."

Romolo Romano reported that the velocity of the lava flow from the main vent, at about 2,280 m altitude on the S flank, continued to decrease in early July, from somewhat less than 0.5 m/s in early June, to an estimated 0.1 m/s on 7 July. The rate of outflow, ~3 m3/s in early June, had dropped to ~0.5 m3/s by 7 July. Lava flowed SE, S, and SW onto the S flank lava field that has accumulated since the eruption began, but the flows were smaller than in previous months and reached a maximum length of 1 km. Between 1,800 and 1,950 m altitude, some flows approached the E edge of the lava field and effusive pseudo-vents remained numerous. Moving lava was visible through many small windows. Between the main vent and the lava field, numerous overflows occurred from the E and W sides of the main lava channel from 2,270 to 2,100 m altitude. As of 8 July, the volume of lava erupted since 28 March was estimated at 100-110 x 106 m3.

J.C. Tanguy reported that on 14 June, the temperature of a slow (~0.15 m/s) lava outflow at 2,280 m altitude was determined to be 1,071-1,073°C using both Cr-Al and Pt-Rh thermocouples inserted at 40 cm depth. This temperature was identical to that measured in the main lava flow 12 May by P. Mousnier-Lompre. Lava velocity at the vent began to increase 14 June and reached almost 4 m/s 18-21 June. Gas pressure increased during the same period and new hornitos were built just below the main vent between 2,280 and 2,260 m altitude. Lava overflows occurred in this zone 18 and 20 June. On 21 June, gas pressure decreased and the lava velocity at the vent decreased to <1 m/s. The level of lava in the main channel dropped again, leaving impressive grottoes where gas combustion produced temperatures as high as 1,137°C (thermocouple) - 1,165°C (infrared measurements) on 21 June.

Moderate to violent explosions have continued to occur from Bocca Nuova. Explosions on 20, 24, and 26 June, and 8 July were especially strong. On 24 June, emission of white vapor had been increasing since at least 0700, culminating in an explosion at 1015 that ejected old material, including reddish cinders and large blocks, that fell as much as 250 m from the vent, primarily to the W. A polar orbiting satellite image 26 June at 0606 showed a low-altitude plume that extended ~100 km ESE, and a similar plume, ~150 km long, trended SSE on 4 July at 0640. Larger plumes were seen on satellite images 8 July at 1427 and 1609 (more than 500 km long, to the ESE), 10 July at 0615 (500 km, to the SSE) and 2036 (150 km, to the SE), and 11 July at 0554 (300-500 km, to the SSE). On 12 July at 1415, the U.S. Navy reported a cloud extending 250 km to the SE with a base at 1 km altitude and a top at ~5 km altitude. Only weak vapor emission has been observed from the Chasm, and from vents at 2,700 m on the S flank active earlier in the eruption.

After 131 days of activity, the eruption stopped during the morning of 6 August. The July activity was similar to that of the second half of June. The main lava channel was almost completely roofed over, but moving lava was visible through four "windows" in the channel roof. Numerous overflows from the upper "windows" produced modest lava flows of short duration during the first 10 days of July. Through the end of the month, lava emerged from scattered short-lived pseudo-vents at about 1,860-1,800 m above sea level and flowed onto the S flank lava field that has accumulated during the eruption (figure 13). These small superposed flows approached the E and W edges of the lava field; one advanced beyond the field's W margin on 13 July but stopped quickly. Efforts to contain the lava flows continued with the construction of new small embankments. None of the July flows moved below 1,600 m altitude.

Figure 13. Summit at S flank of Etna, showing the active vents and lava field of the 1983 eruption. Fractures are shown diagrammatically by short N-S lines. Contour interval 200 m. Large arrow on the upper W side of the lava field indicates the site of the partially successful attempt to divert lava into an artificial channel 14 May (08:05). Embankments constructed to limit the lava's spread are shown by x's. Several roads and villages in the area are shown (Sapienza is an inn, and Mt. Mazzo is an old vent). Nicolosi, Regalna, and Rocca cover larger areas than indicated. Courtesy of Romolo Romano.

Ash emissions occurred at irregular intervals from Bocca Nuova, but were not as strong as in the previous month. High-altitude winds carried ash to Catania (~30 km to the SSE) on 9, 10, and 11 July. No significant activity stoccurred from other vents.

Preliminary estimates suggest that the 131-day eruption extruded ~100 x 106 m3 of lava, at a rate of 10 m3/s. Lava flowed a maximum of 7 km from the vent, reaching 1,100 m altitude (E of Mt. Mazzo), and covered an area of ~6 km2.

"Beginning in December, numerous seismic crises were recorded, mainly connected to the degassing of the magma column through the central vents. In the same period, ejections of reddish ash (old material) or dark ash (fresh material) occurred from the central crater. At times (14, 16, and 28 January) these have been rather significant, depositing thin layers of ash on the E flank. Some nights, pulsating flashes, due to the ejection of incandescent material from Bocca Nuova were observed. Tiltmeter variations were also recorded."

"During the night of 27-28 April, an eruptive fissure oriented approximately NW-SE opened inside the Southeast Crater, near its NE margin. This crater, which is at ~3,000 m elevation behind the SE side of the central crater, formed in May 1971 and had numerous eruptive episodes in 1978 (April-July, August, November), 1979 (July-August), and 1980 (January-September, explosive activity only).

"Initially, moderate activity was observed from three explosive vents along the fissure, which ejected lava fragments. At the same time, small lava flows emerged from the fissure, remaining inside the Southeast Crater. During the next few days, strombolian activity increased with the ejection of lava fragments to 250 m or slightly more in height. During the morning of 1 May, lava flowed over the NE rim of the crater then turned SE, covering the E side of the Valle del Bove and quickly reaching a length of 2 km. Feeding of this flow was continuous but of variable volume, resulting in numerous superposed and parallel subflows but little advance of the flow front after its initial rapid movement. On 6 May, lava overflowed the SE rim of the crater, advancing NE and later E. This flow, which was fed until the morning of 8 May, reached a length of about 1 km. As of 10 May, both the strombolian activity (which formed a scoria conescoria cone inside the Southeast Crater) and the effusive activity appeared to be decreasing.

"Starting 5 May, strong ash ejections from the Chasm were observed, while from Bocca Nuova only emission of gas under pressure was detected. In the past months, a lava lake 150 m from the rim of Bocca Nuova has been noted."

"The Southeast Crater eruption was continuing in early June. The explosive strombolian activity from the small new cone within the Southeast Crater had been diminishing, and stopped almost completely 13 May. Starting that day, ash ejections have been observed at more or less regular intervals, while slow emission of gas and vapor usually occurred. The strombolian activity started again in late May; at times (25 May and 4 June) it was particularly violent.

"The effusive activity has been continuous, with alternating phases of greater or lesser vigor. The lava field has grown noticeably toward the S (reaching a maximum dimension of more than 500 m) and by early June had in its interior, many ephemeral effusive vents, which generated small lava flows that advanced over earlier ones. The main flows (generally one to the S and another to the N), which originated from convergence of the small flows, barely got below 2,700 m elevation.

"At irregular intervals, more or less violent ejections of reddish ash from the Chasm have been noted, while from Bocca Nuova there have only been gas emissions."

Tanguy and Clocchiatti reported that in late May and early June explosions, usually 10-30/minute, ejected lava fragments and scoria to 50-200 m above the inner cone that had formed inside the Southeast Crater. Explosive activity sometimes declined to weak puffs of gas without much tephra, but at other times the ejecta were rich in large fragments of magmatic material. During periods of more vigorous activity, occasional bursts hurled the smallest tephra to 500-600 m height. In contrast, lava effusion occurred at a steady rate of a few (perhaps 2-5) cubic meters per second, significantly lower than in paroxysmal eruptions (>10 m3/s) but probably higher than in typical subterminal persistent activity (1 m3/s). The maximum temperature measured at small effusive vents on 31 May and 5 June was 1,075-1,076°C (at 60 cm depth).

Romolo Romano reported that explosive activity at the Southeast Crater in June and early July was limited to ejection of incandescent tephra at varying intervals, occasionally accompanied by ejection of dark ash. Effusive activity continued, resulting in further enlargement of the lava field, especially to the N. Lava flows did not extend much below the 2,800 m level. During the last 10 days of June, lava overflows occurred from the SE side of the Southeast Crater, but did not flow over the edge of the Valle del Bove. On 4 July, an overflow of very thin (0.5 m thick) fast-moving lava occurred from the still-active SE side of the Southeast Crater. Tanguy and Clocchiatti reported Bocca Nuova was filled with lava to within ~100 m of its rim in April, but by early June the lava column was again very deep (> 300 m) and activity was limited to quiet emission of large amounts of SO2. Romano reported that violent ejection of reddish ash from Bocca Nuova was observed beginning in the second half of June. Recently, the ash has been gray (indicating presence of new material). On 3 and 9 July, violent explosions ejected lava fragments that fell outside the crater rim, especially on the SW flank. During this period, strong emission of gas under pressure was noted at the Chasm.

Tanguy reported that in late April seismicity recorded by Christian Archambault from a geophone about 1 km SSE of the Southeast Crater increased from less than 500 to more than 2,200 events/day (figure 14). A seismic crisis was also recorded January-March, accompanied by lava filling and strombolian activity at Bocca Nuova. The temperature at 3 cm depth at the station about 400 m SSE of the Southeast Crater decreased before the 1983 flank eruption but increased before the current eruption began in April 1984.

Figure 14. Number of seismic events recorded per day (averaged over a 10-day period) at the TDF geophone about 1 km SSE of the Southeast Crater (top) and plot of the difference in subsurface temperature (measured at 3 cm depth) between the CC1 (~400 km SSE of the Southeast Crater) and TDF stations (bottom). Arrows indicated periods of eruptive activity. Courtesy of J.C. Tanguy.

Lava production and Strombolian activity continue from Southeast Crater; strong explosions from Central and Northeast craters

"The Southeast Crater eruption was continuing in early August with more or less intense strombolian activity, accompanied at irregular intervals by violent expulsions of dark ash. This activity produced a scoria cone (~50 m high) higher than the rim of the Southeast Crater. The effusive activity took place from vents around 3,000 m above sea level that changed their positions continuously. On 6 August, two effusive vents were active along the old rim of the Southeast Crater, one on the NE edge, the other on the S edge. Some rather well-fed flows originated from these vents. The final flow direction was always E, toward the Valle del Bove. During this period, the lava flows never advanced below 2,600 m. The lava field that formed from this continuous and variable (in terms of intensity and position) effusive activity was larger than 1 km in extent. The volume of lava emitted can be estimated at around 8-10 x 106 m3.

"An increase in central crater eruptive activity was recorded in July. From Bocca Nuova, violent expulsions of gray ash continued at irregular intervals, while on the vent floor, violent and continuous strombolian activity continued. At times, incandescent lava rose higher than the crater rim. The Chasm, after showing activity similar to that at Bocca Nuova in mid-July, was the source of violent activity on 19 July between 1300 and 1700. Very violent strombolian activity ejected incandescent lava fragments about 1 m in diameter to 500 m from the crater rim. The S and N flanks of the central crater were most often impacted by the lava fragments (their average diameter was ~30 m, they fell within an average radius of 300 m)."

The pilot of an aircraft flying near Etna at 1542 on 19 July observed an eruption cloud that reached ~6.5 km altitude. At 1613, the NOAA 7 polar orbiting satellite showed a plume extending 100 km E from Etna.

"After this, the Chasm remained obstructed until 1 August, when it reopened (at 1900) with the expulsion of old material that fell outside the crater rim. On 6 August, this vent was once again obstructed (around 1300) as the result of internal landslides."

"The Northeast Crater, inactive since February 1981, had a violent explosion that ejected old material on 20 July at 1715. Since then, strong emissions of gases occurred from the small vent that formed near the summit."

"The Southeast Crater strombolian activity was intense at times (23 August, 1 and 6 September) and almost absent at other times (afternoon of 7 September). Violent expulsions of dark ash still occurred, at irregular intervals. The effusive activity took place through various vents along the edge of the Southeast Crater (around 3,000 m elevation). In August the effusive activity occurred mainly on the SE side producing lava flows variable in number, position, and rate of feeding. At first they were directed toward the S; later they turned E, rarely (on 23 August) advancing below the edge of the Valle del Bove (about 2,700 m elevation). On 31 August, one of these flows advanced to ~200 m from the rifugio Torre del Filosofo at 2,910 m elevation. The flow then turned E and stopped 1 September at 2,780 m elevation.

"August activity from Bocca Nuova was similar to that of the previous month, mainly showing emission of gas and steam. The Chasm remained obstructed by landslides within the conduit. During the first few days of September, isolated expulsions of reddish ash from the Northeast Crater were noted, always in the afternoon. The last one was observed on 5 September. Usually, strong emissions of gas occur from this crater."

On 16 August at 0606, a weather satellite image showed a plume extending ~200 km SE from Etna at about 5.5 km altitude. The next morning at 0726, a similar plume was present on the imagery. Low sun angles in the early morning improve the visibility of eruption plumes.

Continuing strombolian activity, particularly intense at times (on the 14th and 20th) and very irregular in intensity and duration, marked the activity in September. Beginning 27 September, this activity was nearly replaced by expulsions of reddish ash at irregular intervals, rarely accompanied by the ejection of incandescent material (bombs and incandescent lava fragments). Lava flows, rarely thick (20 September), usually emerged from effusive vents on the S rim of the Southeast Crater at 3,050 m elevation, and were directed mainly SE, E, S, and ENE. A lava flow advanced S to about 100 m from the rifugio Torre del Filosofo. The flows moved across the lava field, rarely advancing below 2,700 m elevation, within the Valle del Bove.

"Strombolian activity occurred at the bottom of Bocca Nuova. The activity was particularly violent at times with ejection of incandescent lava fragments above the level of the crater rim (24 and 27 September). Generally, only gas emissions were observed. The Chasm usually remained obstructed. Rarely (11 and 24 September), incandescent gas under pressure was observed emerging from a small vent. From the Northeast Crater, isolated expulsions of reddish ash continued, but activity was usually limited to quiet gas and vapor emissions."

Michael Matson reported that a weather satellite image on 14 September at 0740 showed a plume at 4.5 km elevation or higher extending SE from Etna. Ejection of the plume was estimated to have begun 1-2 hours earlier. No plume was evident on an image at 0946.

The following report by Christopher Kilburn, is based on observations 25 August-1 October.

"Since the eruption began in late April, a cone has developed inside the upper E rim of the Southeast Crater, roughly coincident with the position of the most northerly vent active in 1979 (figure 15). On 1 October the cone was ~120 x 100 m at its base (the longer axis oriented roughly NNW) and some 30-40 m high. Until 20 September, the strombolian activity consisted of intermittent violent explosions, with peak frequencies of 14/minute and bomb trajectories ranging from subvertical to directed. Lower-angle ejections were mainly to the E and S, but occasionally to the N and W. Bombs of the order of 0.04 m3 had maximum ranges of ~300-400 m. After 20 September, the violence of the explosions markedly decreased. Weak ejections every few minutes or hours were interspersed with periods of inferred internal collapse that generated small convection columns of pale brown ash and dust. This change in explosive behaviour followed a brief period of rapid lava discharge to the E on 20 September as well as a period of heavy precipitation. Enlargement of the vent at the top of the new cone, which occurred sometime between 13 and 24 September, may have been associated with the 20 September lava discharge.

"Lava discharge was virtually continuous from ephemeral vents S and E of the base of the new cone. Lava flowed S, SE, and E over low points in the Southeast Crater rim. Activity typically alternated between the S and E vents at intervals of one to several days. The successive aa flows, few of which were longer than 1.5 km with frontal thicknesses rarely exceeding 5 m, created a compound flow field with an estimated area that approached 1.5 km2 by 1 October (figure 16). By 23 September, ponding inside the Southeast Crater had reached a maximum accumulated thickness of more than 45 m, raising the level of its bottom to ~3,065 m above sea level. Assuming a conservative average thickness of 5 m, and including the material within the Southeast Crater, a provisional estimate of the minimum volume of the flow field on 1 October is 8.5 x 106 m3, yielding a minimum average effusion rate of ~0.64 m3/s. Estimates of the eruption rate, made near the vent 17-28 September, were 0.5-1.0 m3/s for lava discharged toward the S. The chief uncertainties in these discharge rates are the velocity profiles across the flow surface and with depth, and the thickness of the flow; for the minimum value, parabolic velocity profiles and a flow depth of 3-4 m have been assumed, the maximum (central) surface velocity and lava channel width being estimated in the field.

"The Strombolian activity at Bocca Nuova was accompanied by the emission of large quantities of SO2-rich fumes. Until at least 24 September, magma was probably roughly 100-200 m below the W rim; observations were obscured by fumes. An apparently fresh bomb was found ~20-30 m from the NW rim 24 September, while others, up to 70 cm across, were seen being expelled toward the N and NW 28-29 September to a maximum estimated range of ~100-120 m. It is not known whether ejection of material outside the crater indicated a rise in level of magma within Bocca Nuova or an increase in explosive vigour due to changes in the physical state of the magma, notably in its vesicle content."

After 172 days of activity, the eruption in the Southeast Crater stopped during the evening of 16 October, although activity continued at the Northeast and central craters. On 17-18 October, only violent ejections of ash at rather long intervals occurred from the small cone that had formed inside the Southeast Crater. The ash ejections stopped completely the morning of 18 October. During the last week of activity from the Southeast Crater, there was a gradual decrease in lava effusion, which was limited to vents along the S rim, and ash ejection from the small cone inside the crater. The lava flows were directed mainly toward the SE, generally stopping after only a few hundred meters.

"During the late morning of 16 October, strombolian activity began from a vent at the bottom of The Chasm. Activity was extremely violent during the evening. At times, lava fragments were ejected above the crater rim, falling back within a 100-m radius. The strombolian activity diminished during the night but continued, with alternating phases, through 17 October. During the morning of 18 October, the crater was obstructed by consolidated lava. It reopened 25 October with the ejection of old lava and ash, and more or less intense emission of gas has continued since then. Bocca Nuova alternated phases of slow emission of gas with periods of intense emission of vapor mixed, at times, with mainly reddish ash.

"Ejections of vapor mixed with reddish ash that had started at the beginning of September from the Northeast Crater intensified during the final phase of the Southeast Crater eruption. During the night of 19 October, weak strombolian activity from the Northeast Crater was recorded at irregular intervals. This activity was succeeded by almost continuous ejection of dark ash, followed during the night of 21 October by the collapse of the crater's summit area. This enlarged the central part of the crater and ejected old lava to a distance of about 500 m, mainly toward the W. Discontinuous and more or less intense emission of vapor mixed with reddish ash started 21 October from this crater, while continuous emission of dense white vapor occurred from a small vent to the N.

"During the afternoon of 16 October a seismic crisis began, with earthquakes mainly occurring in the middle and upper parts of the E flank. The strongest shocks (M ~3.5-4.5), which were felt, were all shallow (around a few kilometers)."

A few of the largest earthquakes in the swarm are described below. Date, time, epicenter, and some casualty and damage data are from Romolo Romano; intensity values and the remaining casualty/damage information are from press sources.

19 October, 1843: centered near Zafferana Etnea (11.5 km SE of the summit): 1 person was killed and others injured. Intensity reached MM VII-VIII. More than 400 buildings were damaged, including 50% of the historic district, and about 500 people were left homeless. Damage also occurred in Milo (11 km ESE of the summit), Fornazzo (10.5 km E), Santa Venerina (14 km SE), and Giarre (16.5 km ESE).

25 October, 0211: centered near Fleri (14 km SE of the summit), where it reached MM VIII, injured 12-15 people, destroyed 80% of the houses, and left 900 homeless. Ground cracking was observed in the area. Mt. Ilice, a 350-m-high prehistoric cone roughly 1.5 km upslope from Fleri, lost about 20 m of height during the earthquake. The shock was also felt in Catania.

7 November, 0956, centered near Pedara (15.5 km SE of the summit), where a few buildings were damaged.

No major eruptive activity has occurred since the Southeast Crater eruption ended in mid-October. From the Northeast Crater, emission of white vapor was more or less continuous and consistent. Sporadic expulsions of reddish ash were observed 27 November and 3 December. Ejection of mainly reddish ash observed at Bocca Nuova was particularly violent 22-24 November. Ash ejected 23 November was mainly dark in color, but on succeeding days was mostly reddish older material that had fallen into the conduit. Ash fell on the lower SE flank. Only weak emission of gas and vapor occurred from The Chasm.

Flank seismicity began as the Southeast Crater eruption ended in mid-October (09:10). Isolated tremors continued in November. Both felt and located events were mainly on the N and NE flanks. No additional damage was reported.

Weak strombolian activity started 8-9 March in the Southeast Crater. Lava began to flow from the Southeast Crater the morning of 10 March and advanced E (toward the Valle del Bove), stopping that night as feeding ended. Mudflows were also observed in the Valle del Bove; heavy rains had caused flooding in Sicily during the previous week. On 11 March, fissures opened on the upper S flank (in Piano del Lago Alto). The press reported that temperatures in some of the fissures were high enough to melt plastic at nearby cable car stations. Ash fell on the towns of Acireale and Fiumefreddo (~20 km SE and 20 km ENE of the central crater). During the morning of 12 March, lava emerged from vents that opened at progressively lower elevations (from 2,620 to 2,500 m above sea level) and flowed S and SSW, reaching 2,250 m elevation by the next morning. Numerous mudflows preceded the lava flows.

An 11 March newspaper article citing the National Institute of Geophysics reported that microtremors with epicenters in the W part of the Valle del Bove had been recorded for the past few days. A M 3.4 event near the central crater occurred 9 March at 1523, and a shock with a focus at 5 km depth was felt 10 March at 1101 in the towns of Linguaglossa, Milo, and Sant Alfio (16 km NE, 11 km ESE, and 13 km E of the central crater). No magnitude was reported for the 10 March earthquake but both events were said to reach MM IV-V intensity.

The following is a report from Romolo Romano. Additional information about the eruptive activity from French volcanologists has been inserted in parentheses in this section. Their report of seismicity and tilt associated with the eruption is presented below.

"Explosive and effusive activity occurred 8-10 March from the Southeast Crater (French volcanologists noted that a strong but short-lived phase of the Southeast Crater activity started 10 March at 1100, with vigorous lava fountaining and overflow of lava toward the Valle del Bove). After numerous fractures formed 11 March on the upper S flank between 3,000 and 2,600 m above sea level, an exclusively effusive vent opened 12 March at 2,620 m elevation, with no recorded seismic activity. The same day, three more effusive vents opened at lower elevations (2,600, 2,510, and 2,490 m); only the last two (near Piccolo Rifugio) remained active. The lava flows that originated from these vents moved mainly toward the S and SW, giving rise to numerous individual lobes. Near the vent at 2,510 m elevation, weak explosive activity occurred, soon creating several small spatter cones and hornitos.

"On 14 March, the lava flows moving S destroyed two Etna cableway pylons; the cable broke the next day. The longest flow stopped 15 March at 2,080 m elevation. Along this trend, superposing lava flows were noted during the following days, until the effusive activity from the vent at 2,490 m ceased on 23 March.

"The lava flows that moved SW created an extensive lava field (maximum width 500 m). At 2,100 m elevation the lava field split into at least five lobes. The longest (and easternmost) flow had descended to 1,850 m elevation by 4 April, covering a distance of ~3 km.

"As of early April, the main lava channel had become a lava tubelava tube between 2,450 and 2,300 m above sea level. Short-lived effusive vents opened near the lower end of the tubelava tube, with lava flows approaching the E side of the lava field or flowing over it. The velocity of the lava flows varied from a few meters per hour to ~30 m/hour. The surface covered by the lava can be estimated at around 2.5 km2 and the volume of lava at ~12 x 106 m3. The temperature of lava at the vents was around 1,050°C.

"Particularly during the first few days of the eruption, because of the snow cover, several mudflows formed ahead of the lava flows. Phreatic explosions, violent at times (25 March) were also observed. During the eruption, more or less vigorous emission of vapor, gas, and rarely ash occurred from the central crater vents. (French volcanologists reported a strong explosion from the Chasm on 1 April at about 1625). Isolated expulsions of dark ash from Northeast Crater were observed.

"Collection of information on the eruptive activity was possible thanks to the cooperation of the following Italian Alpine Club rescue team guides and volunteers: G. Baglio, A. Cariola, A. Cristaudo, C. Ferlito, A. Nicotra, G. Puglisi, and F. Zipper."

Later newspaper reports described a swarm of about 30 earthquakes, some reaching intensities of MM IV-V, that started 7 April and continued until early on the 9th. The events were felt most strongly in the towns of Santa Venerina, Acireale, and Giarre (14.5 km SE, 20 km SE, and 17 km ESE of the central craters). After the swarm, the rate of lava production increased by a factor of about 1/3.

The following report, on activity monitored through the ARGOS system, is from PIRPSEV, CNRS-INAG, in the context of French-Italian cooperation.

"Two geophones and a Blum pendulum inclinometer, connected with ARGOS for telerecording of the data, operate near the summit (TDF) and on the south flank (SLN) (figure 17). The geophones record both earthquakes and pulses of volcanic tremor as seismic events, and appear to be good indicators of seismic activity linked to volcanism. The eruption was preceded by an increase in seismic activity (mainly tremor energy) as indicated by the two geophones (TDF and SLN, figure 18). However, after the Southeast Crater eruption, the tremor energy returned to very low levels (figure 19), although effusive vents subsequently opened on the S flank.

Figure 17. Sketch map of the summit and south flank of Etna, showing locations of geophones at Torre del Filosofo (TDF) and Serra la Nave (SLN), and orientation of the tiltmeter at SLN.

Figure 18. Seismicity (number of events per day), top, and tilt, bottom, preceding the 10 March eruption, recorded by the geophones at TDF and SLN, and the SLN tiltmeter. The TDF geophone detection level is 6µm/sec, the three SLN levels are 1.8, 9, and 45 µm/s.

Figure 19. Number of events per hour recorded by TDF geophone, 7-12 March 1985, showing a strong increase during the Southeast Crater eruption. Arrows at top indicated that eruption and the production of flank lava flows.

"The SLN inclinometer showed continuous tilt toward the WSW during early February, consistent with inflation on the site of the 1983 (and March 1985) eruptions (figure 18). Since mid-February, however, tilt has been mainly westward (figures 20 and 21), thus indicating inflation lower on the S rift zone. This change appears almost synchronous with the appearance of new frequencies of volcanic tremor (personal communication by S. Gresta)."

"The S flank activity that began 12 March was continuing in early May. During April, the main lava channel was transformed to a continuous lava tube (with at least three 'windows') from 2,510-2,320 m elevation. Around 2,300 m elevation numerous ephemeral effusive vents formed, variable in number and location, from which several lava flows originated and advanced over the lava field or along its E edge.

"The lava flows, directed mainly SE, S, and SSW, have not advanced much, generally stopping at 2,150-2,050 m above sea level. The lowest elevation reached during this period was 1,950 m (20 April). At times (19 and 21 April and 6 May), because of an increase in the production rate, lava overflows occurred following roof collapses in the upper parts (2,510 and 2,485 m elevation) of the lava tube, giving rise to small lava flows of short duration.

"No gas emission was noted from the mid-April hornitos. More or less intense emission of gas and vapor continued from both of the central crater vents; ash emission was very rare. The Northeast Crater generally emitted vapor and rarely (10 April and 9 May) ejected reddish ash.

"The surface covered by lava can be estimated at around 3 km2 and the volume at ~20 x 106 m3. In this period there has been almost a total absence of earthquakes. However, during the week of 8-14 April, 20 shocks with magnitudes less than 3 were recorded. Afterwards, a variation in the main spectral peaks of the tremor was observed. A gradual increase in the tremor energy was observed during the entire month of April (S. Gresta, personal communication).

"The Etna Guides and volunteers from the Italian Alpine Club rescue team cooperated in the collection of information about eruptive activity."

"The eruption continued through May without significant changes from last month. The main lava channel has been transformed into a lava tube, between 2,510 and 2,320 m elevation, that has at least four windows through which it was possible to see the lava flow. The lava has maintained a constant velocity during the last few weeks. On 23 May another lava overflow occurred as a result of roof collapse along the upper portions of the lava tube (2,485 m elevation), generating small lava flows of brief duration.

"The numerous short-lived vents inside the lava field (from 2,320 to 2,150 m elevation) were variable, as usual, in number and position. The lava flows that originated from these short-lived vents have increased the size of the lava field on both the W and E sides to a maximum width of ~1.5 km.

"Beginning 10 May the lava moved mainly toward the SW (Monte Rinatura and Monte Nero). Around the beginning of June there were numerous lava flows toward the SE (1910 craters area). Lava continued to flow toward the south (Monte Castellazzi area), but these flows were not strongly fed. None of the lava flows descended below 2,000 m elevation.

"The more or less intense emission of gas and vapor from both vents of the central crater and from the Northeast Crater continue. Emissions of ash are rare and inconsistent. Gas under pressure emerged from a small opening at the southern base of the 1984 cone inside the Southeast Crater (P. Briole, personal communication). R. Clocchiatti conducted temperature measurements with a thermocouple; the temperature ranged between 1053°C and 1,088°C (CEN-SACLAY).

"M. Cosentino and G. Lombardo reported that no particular seismic activity was recorded. Instead, an increase in the average amplitude of harmonic tremor was noticed, probably connected to the degassing of the central and Northeast craters.

"The Etna guides and rescue volunteers from the Italian Alpine Club (A. Cristaudo and A. Nicotra) helped with information on the activity."

"After a brief period of quiescence from 11-13 June, the eruption continued. Only one effusive vent remained active (2,485 m elevation) with major lava flows originating from it. From 9 to 11 July, following a brief increase in activity, four effusive vents were present at high elevation (2,490-2,480 m). The velocity of the lava produced in this period ranged from 20 to 7 cm/s, volume 2.5 to 0.5 m3 /s.

"The directions of the lava flows were mainly toward the S, SSE, and SE. In the last few weeks, lava flows of significant size were also present, and were directed toward the SSW, S, and SW. The short-lived vents were still present and numerous, scattered on the lava field from 2,350 to 2,250 m elevation. As usual, they varied in number and location. Recently, the lava field enlarged on the E side, but the lava flows have not descended below 2100 m elevation. The emission of gas continued and was sustained from the central craters, and the Southeast Crater. At times, expulsion of generally reddish ash from Bocca Nuova was noted. From the Northeast Crater, violent expulsions of old material (28 June), and emission of reddish ash (10 July) were observed.

"The temperature of the lava flow at the main effusive vent was around 1,080°C (P. Briole). During this period seismicity remained at low levels. On 12 June at 1848, a seismic event (M 2.5) with an epicenter in the lower E flank (14 km depth) was recorded. On 7 July, three events of low magnitude (<2.8) with epicenters on both the E and W flanks were also recorded.

"On 9 July at 0430, the amplitude of harmonic tremor increased abruptly, coinciding with the increase in effusive activity. During the following 24 hours, the amplitude of the harmonic tremor returned to normal levels (communications with M. Cosentino and G. Lombardo). The Etna guides and rescue volunteers from the Italian Alpine Club have cooperated in the collection of information."

"The effusive activity that had shown new strength during the last days of the eruption greatly diminished on 12 July. Only one main effusive vent at 2,490 m elevation and two short-lived effusive vents in the lava field (at 2,300 and 2,600 m) remained active. The flows from the upper vent never descended below 2,400 m elevation and those from the lava field vents never below 2,100 m. From preliminary estimates, the area covered by lava was ~2.2 km2 and volume of lava produced was ~30 x 106 m3. The maximum width of the lava field is 1.5 km, the maximum length of flows, 1,830 m (figure 22).

Figure 22. Preliminary map of 1985 lava field and its relation to the 1984 lavas. Data, provided by Romolo Romano, is provisional and subject to revision. "Bocca Nuova had a relatively sustained gas emission. Inconsistent expulsions of reddish ash were also observed. This activity was related to explosive activity on the floor of the vent (~300 m below the rim), and was limited to the collapse of the vent's internal walls. The Chasm produced only weak gas emission. Collapses of the walls of the Northeast Crater were observed, and starting 22 July, this crater was partially obstructed, with emission of gas under pressure. Except for the isolated seismic events felt in the upper part of volcanic on 19 and 20 July (M 2.8) and 22 July (M 3), the seismicity remained at low levels. [Originally published in SEAN 10:08, without accompanying text.]

Eruptive activity. (Romolo Romano) "Strombolian activity started from the Southeast Crater on 19 December. The activity occurred at irregular intervals, becoming increasingly intense and continuous in successive days. Since the end of the 12 March-13 July 1985 eruption, more or less intense strombolian activity had been observed from several explosive vents (variable in number and position) on the floor of Bocca Nuova at variable depths (from 100 to 300 m or more from the rim of the vent).

"On 25 December at 0340 an eruptive fissure opened on the W side of the Valle del Bove (on Etna's SE flank), beginning at 2,750 m above sea level. The opening of this E-W-oriented fissure was preceded by a seismic crisis (see below). Strombolian activity soon started along this fissure from at least three explosive vents, while a lava flow began from the lower end of the fissure, covering, in a period of 18 hours, a distance of 1.5 km. The lava flow stopped at a point NE of Monte Centenari, at about 1,700 m elevation, within the Valle del Bove. Eruptive activity from the fissure stopped early the next morning. Activity resumed from the same place early 28 December. This second eruptive phase was characterized by weak strombolian activity from the three explosive vents, and created small spatter cones. The effusive activity decreased; a very viscous lava flow moved ~300 m from the origin, branching at ~2,600 m elevation. The eruptive phase ceased during the early morning of 31 December. A violent expulsion of ash and lapilli from the summit craters (E and W vents of the central crater, Southeast Crater, and Northeast Crater) during the first hours of the eruption was succeeded by more or less continuous and consistent emission of reddish ash.

"The Etna Guides (S. Carbonaro, O. Consoli, A. Mazzaglia, A. Nicotizza, and volunteers of the Alpine Rescue Team of the Italian Alpine Club (A. Cristaudo) have collaborated in collecting information about the eruptive activity.

Seismic activity. (M. Cosentino, M. DiFrancesco, and E. Lombardo) "A seismic crisis began during the early morning of 25 December with shocks located mainly between Piano Provenzana (NE flank) and the Valle del Bove. The shocks were very shallow (2 km or less). At the same time, the amplitude of harmonic tremor increased sharply. The shock that destroyed the hotel Le Betulle at Piano Provenzana, killing one person and injuring seven others, occurred on 25 December at 0338 and had a magnitude of 3.5. During the following 48 hours, about 200 more tremors with magnitudes of 1-4 were recorded. The strongest, M 4, occurred on 26 December at 0334, with its epicenter at Piano Provenzana. Focal depths were 2-3 km. The area of the epicenters remained between Piano Provenzana and Valle del Bove. Beginning 27 December, seismicity decreased in frequency, energy, and number of events, and stabilized to values of ~5-6 shocks/day. Their location was mainly in the eruptive area and on the E and W flanks (especially in early January). Similar activity was continuing on 10 January."

Newspapers reported that continuing seismicity included an event on 12 January at 0037, centered in the Zafferana Etnea area (11 km SE of the summit), that reached MM 6 and damaged some buildings. At least 3-4 minor shocks (one of MM 3) were felt the previous day.

Ground deformation. (G. Nunnari and R. Velardita) "During the second half of December, the tilt stations (Pizzi Deneri, NE flank, elevation 2,850 m; and Serra Pizzuta Calvarina, S flank, elevation 1,650 m) recorded a progressive inflation of the upper E flank. At the Pizzi Deneri station the variation was 13 µrad radially and 22 µrad tangentially between 20 December at 0900 and 23 December at 0900. The same stations recorded a variation of 45 µrad radially and 42 µrad tangentially between 24 December at 0900 and 25 December at 0900. The deformation produced during the first hours of 26 December reached a level that remained substantially unchanged as of 10 January."

Seismicity has continued since Etna's SE-flank fissure eruption in late December. The following is from R. Romano. "Beginning 1 February, a series of shocks occurred on all ... flanks (table 2). The most energetic events occurred 1 and 2 February ~ 15 km SE of the summit (in the Dagala zone, in the area of Santa Venerina and Linera), with magnitudes between 3.0 and 3.6 and depths within 1 km. During the week, 50 shocks were also recorded by instruments, mainly on the E flank."

Table 2. Largest of a series of earthquakes on Etna's flanks, February 1986.

While spending the night of 5 March at the foot of Northeast Crater, geologists observed ash emission every 5-15 minutes with ejection of glowing red bombs 6-7 times/hour. Bombs reached the base of the crater. Significant degassing occurred from both of the central craters (The Chasm and Bocca Nuova). However, there were no explosions, nor was glow visible at night.

Explosive activity has continued from both central crater vents since the end of the SE flank fissure eruption. Strombolian activity could be seen from within ~ 100 m of the W vent (Bocca Nuova) rim, but ejected lava fragments rarely rose above the crater rim. Active vents within Bocca Nuova varied in position and number. Since the end of April, a vent on Southeast Crater has emitted hot pressurized gas. Northeast Crater has been partially obstructed by internal landslides; weak fumarolic activity occurred along the crater walls.

Strombolian activity and lava production, late July-23 September. Strombolian activity began from Northeast Crater at the end of July and continued with periods of greater (19 August) and lesser intensity. In early September, 20-30 explosions occurred/minute, with ejection of bombs and scoria to 100-250 m above the vent. A scoria cone ~ 40 m high formed inside the crater. Strombolian activity increased in the late afternoon of 13 September, and lava overflowed Northeast Crater's W rim early the next morning, feeding a modest-sized lava flow that moved NW. By 16 September, the flow had descended to 2,920 m altitude (~ 1.5 km from the main effusive vent at 3,190 m altitude) after crossing a trail (maintained by the Società Turistica Star). During the following days, small lava flows advanced a few hundred meters NW, NNW, WNW, and W, beside or on top of the first flow. This slow and discontinuous activity built a small lava field, with a volume that was estimated at a maximum of 0.25 x 106 m3. On 22 September, the temperature of the flowing lava near the vent was 1,094°C at 40 cm below its surface. During the effusive phase, Strombolian activity varied in strength but generally remained at rather low levels. While sampling high-temperature gases and measuring oxygen fugacity on 22 September, Patrick Allard and others noted that the floor of the crater oscillated, rising by as much as 1 m with every explosion. During a summit-area leveling traverse the next day, a very strong tremor made automatic level readings difficult. Strombolian activity increased considerably, with bombs rising to 200 m above the vent and falling 300 m away. Strombolian activity also occurred at the bottom of the E and W vents (The Chasm and Bocca Nuova) of the central crater. This activity was of variable intensity, with changing numbers and locations of vents. Only rarely were lava fragments ejected above the vent rims.

Cessation of activity, early 24 September. During the early morning of 24 September, the scoria cone and a portion of the wall inside Northeast Crater collapsed into the vent area, leaving a fuming pit ~50 m deep. Effusive and explosive activity ended almost immediately, between 0600 and 0700. That morning, activity was limited to expulsions of reddish ash and white vapor. Steaming fissures and a small graben (~ 1 m wide and 1 m deep in places, wider nearest the crater) opened to the SW and NE (figure 23), with fissures reaching the Valle del Leone (1.5 km ENE of Northeast Crater) and the Piano delle Concazze, 2 km to the ENE. Activity from Bocca Nuova had also ceased. A leveling team in the summit region and N of Northeast Crater detected some intermittent tremor and measured substantial deflation that had occurred since the previous day.

Figure 23. (left) Sketch map of Etna's summit area, showing the new fissures and graben of 24 September and the area of 24 September bomb fall. Numbers along the route taken by observers show positions of (1) Allard, Benhamou, and Pennisi; (2) Bond, Décobecq, Kilburn, Murray, and Obreski; and (3) Sagot. (center) Provisional sketch map showing depth and percentage of ejecta cover after the 24 September explosion. (right) Maximum particle size with distance from Northeast Crater. Courtesy of J. Murray, C. Kilburn, D. Décobecq, and A. Bond.

Because of the abrupt cessation of summit activity and the possibility of violent vent-clearing explosions, the summit area was closed to tourists.

Resumption of explosive activity, afternoon of 24 September. A prolonged period of ash ejection accompanied by thunder and lightning started at about 1215, causing a light ashfall on the upper SE flank (figure 23). At 1312, large dark ejecta were seen rising to ~ 40-50 m above the vent, with stronger expulsion of small brown ash clouds, but no discrete explosions were heard. Episodic eruptions continued for the next 3.5 hours. Periods dominated by quiet emission of pink-brown ash alternated with periods characterized by ejection of black cypressoid and columnar ash jets, and expulsion of large tephra to ~200-350 m above the vent (figure 24). Electric discharges were common. Thick white convecting vapor clouds emerged separately from the S part of the vent area, at increasing rates. Continuous tremor and occasional stronger shocks were felt after 1515 by a gas sampling team working ~ 200 m S of Northeast Crater. There was no apparent time relation between the seismicity and eruptive activity.

The following preliminary observations have been extensively updated by J.B. Murray.

Ash emission began to increase gradually at about 1645, changing to violent phreatomagmatic explosions. Black clouds, again cypressoid and columnar, initially rose to 400 m above the vent, merging by 1743 to produce a convective cloud that reached a relatively consistent height of ~ 1,000 m. Stronger falls of ash and small lapilli occurred SE of the summit, and bombs reached heights of 600-700 m. No [sounds] were heard, [apart from frequent dry cracks of] electrical discharges in the cloud [and the rushing of air in the rising column]. Incandescent ejecta were first seen at about 1800, and within 40 minutes lava fountains had developed at the center of a much wider column of dark ash. Heat from the fountains could be felt from a distance of ~ 800 m. ... Between 1815 and 1843, bombs fell progressively farther SW from the vent, with maximum ejection distance increasing from ~300 to 700 m, and maximum heights building from ~600 to 1,500 m.

Paroxysmal explosion, evening of 24 September. The paroxysmal phase began about 1845 with continuous louder rumbling and the sudden rise of a vertical lava fountain, ~ 300 m across, to 800-1,000 m. A dense black cloud surged down the E flank of Northeast Crater into the Valle del Bove. [Dense bombs up to 50 cm in diameter fell as far as 2,700 m to the SW, suggesting to Murray that they reached 2-3 km height]. ... The eruption cloud moved SSE, and ... the tephra fallout [rapidly] advanced ~2 km, showering observers [fleeing in vehicles] on the S flank (near the Torre del Filosofo; figure 23) with a dense blanket of ... scoria and ash [with occasional heavy bombs falling from 2-3 km height]. In a short time, the entire SE flank of the volcano was covered by pyroclastic material of sizes that varied with distance from the vent (figure 23). [Differential winds at higher altitude carried some scoria SW as far as Biancavilla (~ 15 km SW), where fragments measuring a few cm fell]. Tephra fall continued for almost 20 minutes.

[Subsequent topographic mapping] showed that pyroclastic material (scoria, lava fragments, and lithic blocks) [reached 4 m thick near the edge of Northeast Crater, and 5 cm thick] at a distance of 2 km (Torre del Filosofo). Scoria and lithic blocks up to a meter in diameter could be found within a radius of 0.5 km; at 2 km, scoria a few tens of centimeters across were observed. Hot scoria 11 cm in diameter melted roof tar 4 km S of the crater (at Piccolo Rifugio); hot lapilli to [7.5] cm across [scratched car windshields at] the tourist complex around Rifugio Sapienza and the base station of the cable car system 6 km from the vent; some scoriae as large as 15 cm were found 7 km from Northeast Crater at 1,700 m altitude (Serra La Nave); and 1.9-[cm] lapilli fell at Nicolosi (17 km to the SSE). Lapilli [0.5 cm in size reached Catania, 27 km away, where light ashfalls also occurred]. Catania airport (35 km from the summit) was closed from that evening until late the next morning.

[Bond, Décobecq, Kilburn, Murray, and Obreski] estimate that the tephra column reached 10-13 km altitude during the paroxysmal phase, in agreement with an independent estimate by Mueller [from Nicolosi. However, photos by J.P. Delouche, approaching the volcano from Siracusa, show maximum column height to be 6-7 km above the summit (9-10 km altitude)]. The press reported that the tephra column was visible from the Aeolian Islands (95 km N), and Agrigento (100 km WSW). At [1900] on 24 September, the explosive activity ceased completely. During the following days, Northeast Crater slowly emitted gas, vapor, and (rarely) reddish ash.

[Nearly 200 depths measured by Murray, Décobecq, and Bond yield a tephra deposit volume of 2.6 x 106 m3, mostly of pumice density, or 0.4 x 106 m3 dense rock equivalent (DRE)]. The volume of material emitted was estimated by IIV geologists at around a few million cubic meters ... . If most of the tephra were assumed to have been erupted during the final phase, the mean paroxysmal eruption rate was [~500] m3 DRE/s (2.7 x 106 kg/s). Kilburn notes that the independently determined values for eruption rate and column height are consistent with the column height model of Wilson et al., 1978. Total energy release was [~9 x 1021] ergs. Vigorous explosive eruptions are relatively uncommon at Etna; [this century only four comparable events have occurred, in 1917, 1940, 1947,and 1960].

Minor activity, late September-early October. Strombolian activity from the two central crater vents, which ended 24 September with the collapse in Northeast Crater, resumed on the morning of 29 September. Gas, at times under pressure, was also emitted from Southeast Crater and a nearby gas vent before, during, and after the paroxysmal explosion. Emission of gas with fragments of old lava and/or incandescent lapilli was observed at times. The frequency of these ejections was very irregular, varying from a few minutes to several hours. As of 9 October, no significant changes in the activity of the summit craters have been noted.

Seismicity. Beginning 13 September, the seismic activity consisted only of explosion earthquakes probably related to the onset of the increased Northeast Crater activity. During the night of 22 September, three isolated seismic shocks with a maximum magnitude of 2.9 were recorded on the NW flank. Minor shocks were recorded until the night of 2 October, when a swarm of 30 events (maximum magnitude 3.3) occurred on the lower NW flank (between Maletto and Randazzo) at variable depths of ~ 20 km. A second swarm started during the morning of 5 October, again on the W flank, at the same depth. The strongest shock (M 3.8) occurred at 1228, and was felt at numerous locations on the volcano. The swarm ended that night, after ~ 40 weak shallow events were recorded on the W flank (Monte Minardo area). Another swarm of ~ 10 shocks occurred the morning of 7 October, also on the W flank, with a maximum magnitude of 3.3.

Harmonic tremor increased during the first phase of Northeast Crater eruptive activity (13 September). Similar tremor energy values were observed until ~ 5 hours before the 24 September eruptive event, when tremor energy began a gradual increase of about an order of magnitude. Energy values returned to normal around 2000 and no significant variations occurred in the following days.

A new system of eruptive fissures opened at the base of the central crater on 30 October. The new eruption was marked by vigorous Strombolian activity and intense seismicity. Several new lava flows emerged from the fissures.

The seismic activity observed during the first several days of October continued during the following days. Both shallow and deep earthquakes occurred, mainly on the W flank of the volcano. The deepest earthquakes were at 20 km and maximum magnitude was 3.0.

An intense but brief seismic swarm that included > 40 shocks in one hour began at 2345 on 29 October. The strongest event (M 3.7), centered on the NE flank (at Piano Provenzana), occurred ~1/2 hour later at 0018. Only a few events were recorded during the following days.

Harmonic tremor remained at late September-early October levels until 30 October, when the seismic swarm was joined by a brief, ten-fold rise in tremor amplitudes, between 0000 and 0400, that accompanied the opening of a system of eruptive fissures. The 2-km-long fissure system, oriented ENE, stretched from the base of the central crater to the Valle del Leone (from 2,900 to 2,500 m above sea level (asl), on the W flank of the Valle del Bove). Several explosive vents that formed along the upper part of the fissure system were the source of gas and vapor emission, strong ash expulsions, and occasional phreatic explosions. Weak Strombolian activity occurred from the lower part of the fissure system (~ 2,500 m altitude) and several lobes of lava flowed away from the fissures. Eight hours after the start of the eruption, two main flows were moving SE. The southern flow had advanced ~ 2.5 km and reached the Valle del Bove at 1,750 m altitude.

During the early afternoon the fissure system continued to propagate downslope, creating a new eruptive fissure NW of Mt. Simone, around 2,300-2,200 m altitude, just behind and to the side of the N "wall" of the Valle del Bove. Violent explosive Strombolian activity started immediately, as four eruptive vents formed. Lava flows from the two lower vents coalesced into a single lobe that moved SE. In the late evening, strong lava fountaining began along a few hundred meters of the central part of the fissure system (around 2,600-2,500 m altitude). The ash and lapilli from this activity were transported by the wind 30 km to the SW.

Similar explosive activity was continuing the next morning. Lava formed three main flows (around 2,700 m altitude), that moved SE and ESE. Tremor amplitude decreased significantly, but increases were later recorded during various periods of increased eruptive vigor. Strong Strombolian activity from Southeast Crater began in the early afternoon. Isolated but violent explosions continued until the next day (1 November) when lava overflowed from Southeast Crater and moved SE for a few hundred meters. Southeast Crater lava effusion ended during the morning of 2 November and ash expulsions, sometimes violent, began.

During the first few days of November, violent explosions audible on the entire SE flank marked periods of intense ash expulsion from the new eruptive vents (particularly those around 2,300-2,200 m altitude). The numerous lava flows remained above 1,500 m altitude, forming a large lava field.

Strombolian activity from the bottom of the 2 central crater vents had continued through October. From the beginning of the 30 October eruption, vapor emission of fluctuating intensity alternated with expulsions of significant amounts of dark ash from these vents. Fumarolic activity occurred along the edges of Northeast Crater.

On 3 November a M 3.5 earthquake, the strongest since 30 October, occurred at 0833 in the lower part of the Valle del Bove. As of the morning of 10 November, weak Strombolian activity was occurring from six explosive vents, two around 2,500 m altitude, and four around 2,300-2,200 m. Lava from the two eruptive systems merged and formed a single flow. It advanced 200 m past Rocca Musarra (on the N side) and penetrated farther inside the Valle del Bove than any other flow, to 1,450 m. At least two large scoria cones were forming over the vents.

"The eruption ... was continuing in early December. As of 9 December, two eruptive vents were still active, as was a pit crater at ~ 2,850 m altitude (figure 25) that generally showed strong gas emission but at times (15 November) violent expulsions of ash and/or sporadic ejection of incandescent material. A substantial spatter cone has formed at the eruptive vent at 2,600 m altitude, from which strong gas emissions have occurred since mid-November. The main lava channel originated at the base of the cone, and since the end of November has been an essentially continuous lava tube (with at least seven skylights in its roof) down to 1,900-2,000 m altitude, where a few ephemeral effusive vents have formed. Several lava flows fanned out from these vents, rarely going below 1,500 m and remaining within the Valle del Bove. The more active lava flows have generally moved E. On 19 November one of these reached 1,325 m altitude, ~ 5 km from the vent.

"A large spatter cone (called Monte Rittmann in memory of our beloved teacher) formed around 2,300-2,350 m, and reached a height of ~ 100 m. From its base, a lava overflow started at the beginning of December and fed a flow that advanced ~700 m before stopping behind Monte Simone. Monte Rittmann has shown almost continuous Strombolian activity, more or less violent and often accompanied by strong explosions. Expulsions of ash have also been observed. The eruptive vents at lower elevation have not been active since 25 November. These vents, one exclusively explosive (at ~ 2,250 m altitude) and two primarily effusive (2,200 and 2,190 m altitude) were very active during the first weeks of the eruption, helping to form a large lava field. The flow fronts were generally wide (at times > 100 m) and high (at times > 10 m). An initial estimate, still to be confirmed, suggests that the volume of lava is around 25 x106 m3. It is difficult at this time to evaluate the surface covered by the lava, but it should be > 4 km2.

"From the E vent of the central crater (the Chasm) more or less violent emissions of reddish ash have continued without interruption. Only emissions of gas and vapor were recorded from the other summit-area vents.

"Only a few weak earthquakes were recorded during November and the first few days of December. Volcanic tremor remained at high levels (5-10x normal), with variations related to changes in the eruptive activity."

"The eruption was continuing in early January. Strong gas emissions continued from the eruptive vent at ~ 2,600 m altitude. Lava continued to flow into a tube that extended from the base of the vent to ~ 1,800 m altitude, where there were ephemeral effusive vents that varied in number and position. The lava flows that originated from these ephemeral vents did not advance below 1,500 m altitude, and remained in the center of the Valle del Bove.

"The Strombolian activity at Conetto Rittmann (2,350-2,300 m altitude), at times from as many as three explosive vents, included periods of greater or lesser activity, but diminished gradually. From the beginning of January, the Strombolian activity was almost totally replaced by more or less violent explosions of gray ash. Lava flows originating at the base of Conetto Rittmann (recently more than two lobes have been noted) usually reached 1450 m altitude, passing N of Rocca Musarra, where they stagnated. The rate of lava production varied considerably with time. At the pit crater at ~ 2,850 m altitude, only gas emissions were observed. More or less violent explosions of reddish ash continued from the E vent of the central crater (La Voragine) alternating with periods (more rare) of vapor emission. Gas emissions from the other summit vents (Bocca Nuova, and Southeast and Northeast craters) were continuous but of varying strength.

"During this period, no seismic shocks were recorded. Volcanic tremor remained at rather high levels, with sporadic variations associated with the trend of the eruptive activity."

"The eruption ... continues. Gas emission and rarely (12, 13, and 18 January) strong expulsions of ash occurred from the pit crater at ~ 2,850 m altitude. Only gas emission was observed from the spatter cone at ~ 2,600 m altitude, and it appears that lava effusion from its base ceased at the end of January.

"The Strombolian activity from Conetto Rittmann (2,350-2,300 m altitude) has decreased gradually, almost completely replaced since mid-January by more or less violent expulsions of gray ash, and irregular intervals in which only gas emission was observed.

"The effusive activity that originates from the base of Conetto Rittmann continued with alternating phases of greater or lesser intensity. Between 9 and 23 January several lava flows (four at times) were noted. These were fed at a moderate rate and descended to 1,500 m altitude, after passing Rocca Musarra to the N and S. As of mid-February, a lava tube extended from the base of Conetto Rittmann down to 2,000 m altitude. From there, small lava flows originated from many ephemeral vents, variable in location and duration. These flows moved mainly SE and E, covering lava from previous days, but remained inside the Valle del Bove, and never descended below 1,700 m altitude.

"Expulsions of reddish ash were observed at irregular intervals from both vents of the central crater (mainly from the E vent), while from Southeast Crater only gas emission of variable intensity was recorded. Weak fumarolic activity occurred from Northeast Crater.

"During the first two weeks of January no earthquakes were recorded. Tremor remained at high levels until the morning of 16 January, when tremor energy was reduced to 1/4 that of the preceding days. Seismicity resumed 27 January and continued for 5 days with weak, sporadic shocks on the N flank, at ~ 10 km depth. At the same time, tremor energy descended to levels typical of quiet periods at the volcano.

"Two strong earthquakes (M 4.2, depth 20 km) were recorded on 2 February at 1648 and 1659. Epicenters were ~ 40 km NW of the volcano and the events were felt in much of eastern Sicily. No additional significant earthquakes had occurred as of mid-February, and tremor remained at low levels."

"The eruption that began 30 October 1986 ceased at the end of February, probably on the 27th, after 120 days. The effusive activity that had originated from Conetto Rittmann diminished gradually. Only two lava flows, poorly fed, were noted around 20 February. These were moving across the lava field that had formed in previous months within the Valle del Bove, and did not advance below 1,800 m altitude. At the end of February, only gas emission, very vigorous at times, was noted at Conetto Rittmann. More or less impressive gas emission, mixed at times with a little ash, occurred from the two central crater vents. Consistent ash expulsion was rare. Gas emission continued from Southeast Crater. During the eruption, Northeast Crater had shown weak fumarolic activity, but emission of gas that sometimes contained a little ash was noted in early March.

"After the two earthquakes of 2 February, no significant shocks had occurred in the area as of 6 March. Throughout February, tremor energy remained at the low levels reached at the end of January. The presumed date of the end of all eruptive activity (27 February) is in accord with a decline in tremor energy, when values reached those typical of quiescence at the volcano."

An explosion from Southeast Crater on 17 April ejected tephra that killed two people and injured seven others.

During the first half of March, Northeast Crater occasionally emitted gas and vapor, sometimes with ash. During the second half of the month only weak fumarolic activity occurred from Northeast Crater, but more or less intense gas emission episodes that occasionally ejected ash occurred from the central crater's Bocca Nuova. Low-level volcanic tremor occurred during the same period but no earthquakes were recorded. Ten microshocks (M 1.0) occurred 23-25 March.

Sudden increases in sporadic tremor, lasting ~ 10-15 minutes, began 1 April. Geologists attributed the tremor to deep phreatomagmatic explosions. Activity increased during the first few days of April, reaching a maximum of 10 episodes of sporadic tremor on the 6th. The episodes lasted 30-40 minutes each and occurred at ~ 2-hour intervals. On 8 April > 50 microearthquakes were recorded. Only weak gas emission occurred from the central crater's E vent through early April. However, on 8 April at 0835 a violent phreatic explosion from that vent fed a 1-km-high eruption column. Abundant tephra was strewn to 300 m from the crater rim, with maximum dispersion to the NE. Geologists believed that similar explosions had probably occurred during the second half of March. The 8 April explosion was followed by a long period of relative seismic quiescence when only weak sporadic tremor was recorded. Beginning 12 April, 4-5 episodes of sporadic tremor were recorded daily.

Following several days of forceful gas emissions from Southeast Crater, tremor duration increased to a maximum of 30 minutes on the morning of 17 April. At 1335 a moderate-intensity phreatic explosion launched tephra SSE, killing two and injuring seven of the ~ 30 tourists who, the press reported, were standing ~500 m from the crater. A similar explosion on 12 September 1979 had killed 9 tourists and injured 23 others near the central crater's Bocca Nuova (04:09). The 17 April ejecta appeared to be older volcanic material. Tephra fragments 150 m from the crater rim reached diameters of 15 cm and at 250 m were a maximum of 5 cm. IIV geologists suggested that conditions for the 8 and [17] April explosions resulted from the collapse of the internal vent walls and subsequent gas accumulation.

Immediately after the [17] April explosion an increase in tremor was noted. The next day, three episodes of periodic tremor were recorded, each lasting ~ 90-120 minutes. Levels of tremor 2-3x normal continued through the end of the month. Only small quantities of gas were emitted from the central crater during the days following the fatal explosion. Weak Strombolian activity [from Southeast Crater] was observed during the night of 25 April and the morning of 26 April [and 10-16 May]. Vapor emission resumed after the end of the Strombolian activity.

The following is from a joint French (CNRS/PIRPSEV)-Italian (CNR) monitoring program.

Ground deformation and microseismicity data are transmitted by an ARGOS satellite station, 1 km SSE of the central craters at 2,920 m asl (near the Torre del Filosofo). Two tiltmeters and a vertical geophone are installed inside a 50-m-long lava tube.

From November 1987 through January 1988, no ground deformation was detected and microseismicity was at a low level (100 events/day). In February, ~ 3 µrads/day of nearly continuous inflation was measured by the N-S component of the tilt instrumentation, but E-W tilt remained essentially flat (figure 26). Microseismicity remained at a low level until 25 February, when both the number of events and noise amplitude began to increase (figure 27). A sharp (30 µrad) N-S deflation occurred 29 February-1 March, with a further increase in the number of events (to 700-950/day) and noise amplitude.

Figure 27. Number of seismic events/day with amplitudes >4 Nm (top), and daily mean value of noise energy (bottom), measured at site TDF, 1 November 1987-12 April 1988. Courtesy of the joint French-Italian Etna survey program.

Seismicity continued to build 1-5 March, to 1,100 earthquakes/day. An additional 15 µrad of N-S deflation 5 March was accompanied by the onset of 20 µrad of rapid uplift to the E. From 5 March through early April, NNE-trending inflation has continued at ~ 4 µrad/day, and microseismicity has remained strong, reaching a maximum of 2,700 shocks/day on 8 April. Since the end of February, the 10-station ARGOS seismic network has located many shallow earthquakes in the NE quadrant of the volcano. Local and regional networks have recorded many earthquakes on Sicily's E coast, especially near Etna. Some reached M3 with hypocenters deeper that 10 km. One was felt during the night of 1-2 April by residents of Santa Venerina (15 km SSE of the summit) and Guardia-Mangano (18 km SSE). The ground deformation and microseismicity data suggested to the French/Italian team that significant stress modifications had been located beneath the central craters, and these have since moved to the E flank. No magma had reached the surface as of early April and degassing activity from Bocca Nuova and Southeast Crater remained normal.

After the October 1986-February 1987 eruption, activity has been confined to the summit craters. In the following report, frequent observations by IIV personnel with help from Italian Alpine Club guides are supplemented by data from geologists who reoccupied ground deformation networks 20 May-24 June and 16 September-7 October.

Bocca Nuova. Since the end of April 1988, Strombolian activity and/or degassing has continued at varying intensity. Explosive vents were active at different points on the crater floor, 150-250 m below the rim. Lava fragments only rarely reached the crater rim. Ash and lapilli ejections were occasionally observed, especially during periods of stronger and more continuous activity. In May and June, small explosions or gas bursts could be heard deep within the crater at a rate of 30-50/minute, with larger explosions about every 5 minutes. Loudness of the explosions varied considerably; they usually were not audible 400 m from the crater, but on 23 June, 4 were heard from Rifugio Sapienza, 5.5 km S, between 1214 and 1226. In September and October, the explosion rate was 30-40/minute. Strombolian explosions from three active vents on the crater floor, roughly 150 m below the rim, were observed about once every 5 minutes. Louder series of explosions were heard up to 3 km away on 23 and 28 September and 1 October. During the night of 1-2 October, a few fresh bombs landed just outside the crater rim and the next day bombs were rising to ~ 10 m below the rim. Violent activity that included lava fountaining was observed from a distance on 31 January 1989 from 1930 to 2100. Bocca Nuova was relatively quiet in early February.

La Voragine. The vent was partially obstructed until July, with activity limited to weak gas emission. Strombolian activity of variable intensity then started at the bottom of the crater, at more than 250 m depth. Ejecta only rarely reached the rim. On 23 August between 1610 and 1650, a series of pyroclastic explosions ejected juvenile material (lava fragments, scoria, lapilli, and ash) to a few hundred meters height. The tephra fell mainly on the E flank, to 400 m from the rim. Explosive activity ceased completely from the end of August until the beginning of September. During visits on 28 September and 6 October, Strombolian explosions were occurring from vents that appeared to be higher in the crater than those at Bocca Nuova. About 30 explosions/minute were counted on the 28th, with ~1/minute strong enough to eject bombs visible to the geologists, who could see 50-80 m into the crater. At night on 2 October, no noises could be heard from the rim, although faint glow was visible. In early February 1989, little activity was occurring from the crater.

Southeast Crater. Gas emission, sometimes under pressure, continued from a vent near the center of this complex of cones and depressions. By 4 October the gas emission had strengthened, ejecting blocks, apparently from the wallrock, and sublimates. Weak and irregular Strombolian activity began in early October. A small new vent opened between 24 and 25 November E of the previous vent, ejecting ash and other tephra. Some of the explosions were fairly vigorous, as on 27 November. The activity changed gradually from ash ejection to a nearly continuous explosive Strombolian activity, with periods of greater and lesser intensity. At times, lava fragments reached 250 m height, falling within a few hundred meters of the crater. Similar Strombolian activity was continuing in early February 1989.

Northeast Crater. Only fumarolic activity was observed until the end of July, varying in intensity with weather conditions. A small vent opened inside this crater on 24 July and grew with time. Very hot pressurized gases emerged, particularly soon after the vent opened. The vent was glowing at night in September. Smoke rings were occasionally emitted in September and October, similar to those observed from Bocca Nuova early in its development.

The following deformation and seismic data are from J.B. Murray.

Despite the increased seismicity and deformation recorded February-April 1988 (13:03), the June levelling traverse showed an unusually small amount of movement since September 1987. Subsidence of recent lava around Northeast Crater was <2 cm, an order of magnitude less than usual and the smallest recorded since the line was installed in 1975. Inflation of 1.6 cm measured 3.5 km NE of the summit was also unusual. Recently active areas generally showed subsidence, with downward movement of slightly > 1 cm at the 1983, 1985, and September 1986 dikes, and isolated larger movements of -6.6 cm at the 1986-87 fissure and -19 cm at the edge of Southeast Crater. The very large amounts of subsidence measured in recent years in the Valle del Leone have diminished considerably.

A very different pattern was measured during the September traverse, with broad inflation centered just SW of Bocca Nuova. Inflation extended over the entire traverse, reaching +5.3 cm at the center when compared to the southern reference station 2.5 km away, or +8.5 cm compared to stations at the N end of the traverse, 5 km away. This inflation was the largest recorded at Etna when no eruption had occurred between measurements. Most dry-tilt stations showed typical small, more or less random, tilts of apparently local origin. However, 4 stations 4-9 km to the SW showed westerly tilts of 20-30 µrad, suggesting that inflation may extend farther in that direction. Murray noted that summit inflation preceding summit eruptions was measured in May 1976, June and September 1977, September 1978, July 1980, October 1983, September 1984, and June 1986. Inflation episodes centered just SW of the summit have tended to precede Southeast Crater activity.

Small local shocks that were apparently related to summit activity were detected optically during measurements of the levelling line and dry tilt stations. Using the automatic level, relative amplitudes were obtained by noting the amount of crosshair movement against the levelling rod and correcting for distance. In June, seismicity was very quiet, with only four events noted in a week of levelling, all within 450 m of Bocca Nuova. The largest had an amplitude of ~ 60 µrad.

In September and October, seismicity had increased dramatically. On 23 September, 73 shocks were noted in 1 hour while levelling W of Bocca Nuova at a mean distance of 330 m from the rim. The shocks were much stronger than in June, with largest amplitudes reaching 250 µrad. Seismic events were detected as much as 4.6 km from the summit. W of Piccolo Rifugio, ~ 3 km from the summit, three shocks were detected in 1 hour, the largest of 35 µrad.

Summit activity. (S. Calvari, M. Coltelli, and M. Pompilio.) Vigorous Strombolian activity at the two central craters (Bocca Nuova and La Voragine), persisted through March and April. At Southeast Crater, Strombolian ejections became strong and continuous during the last 10 days of March, similar to those of 24 December-24 January and 29 January-22 February. Strong explosions continued through April. A small lava flow emerged from the base of Southeast Crater's cinder cone on 15 April at 2300 and flowed over the crater's S rim. Lava effusion stopped early the next morning. Degassing continued at Northeast Crater.

Seismicity. (E. Longo, A. Montaldo, M. Patanè, E. Privitera, and S. Spampinato.) A total of 71 tectonic earthquakes (M >1.0 and S-P <5 seconds) were recorded on the Serra Pizzuta Calvarina seismic station (~ 13 km S of the summit craters) in March, compared to 194 events during all of 1988. The highest seismic energy release and the largest number of events (9, with two of M 2.7) occurred on 17 March. Tectonic events were fewer and smaller (M<=2.7) in April than in the first three months of 1989. Most of the largest events (M>=2) were located on the W flank at depths of ~ 15-25 km. High-energy volcanic tremor episodes were recorded 9-16 April and represent the only seismic evidence of the Southeast Crater eruptive episode on 15 April. After the effusive episode, the spectral amplitude of tremor greatly decreased, but remained higher than in March.

SO2 emissions. (T. Caltabiano and R. Romano.) During the first half of March, SO2 emissions from the summit craters were similar to relatively high February values at ~ 6,000 t/d. During the last half of March, SO2 emissions increased to ~ 8,000 t/d, then decreased to ~2,000 t/d. After the lava effusion of 15-16 April, SO2 emission rates rose to ~12,000 t/d, but decreased to ~3,000 t/d during the remainder of the month.

Summit activity. (S. Calvari, M. Coltelli, and M. Pompilio.) Vigorous activity at the two central crater vents (Bocca Nuova and La Voragine) continued in May. On the 4th, La Voragine ejected bombs and lapilli that fell as far as the rim of Cratere del Piano (roughly 300 m away), choking the crater bottom with tephra. In late May, explosive activity diminished and continued at a normal level throughout June. Discontinuous effusive activity was observed in May within Bocca Nuova, and bombs accumulated in the crater to ~ 100 m from the rim. From late May through most of June, many bombs, some of considerable size, fell outside the crater. This activity suddenly stopped in late June, when the small cone inside the crater collapsed, and was succeeded by sporadic scoria ejection from two vents. Mild Strombolian activity at Southeast Crater in May slightly eroded the scoria cone that had formed in April (14:05). Strombolian activity continued at a medium-low level in June, with occasional pulses ejecting small numbers of bombs over wide areas. The vent on Northeast Crater's floor continued to degas through May and June.

Seismicity. (V. Longo, A. Montaldo, M. Patané, E. Privitera, and S. Spampinato.) The frequency of tectonic seismicity in May and June was generally similar to that of the past year, with occasional seismic swarms. During the last two days in May, low-energy events were detected ~ 10 km below the volcano's central area. A seismic swarm, recorded 19-24 June on the W flank, was 13-15 km deep and included the largest events (M 3.1-3.2) of the month. One of the earthquakes (on the 24th at 0230) was felt by area residents. On 28 June, a small mainshock-aftershock sequence (11 events) was recorded, with the largest earthquake located near the S portion of the Valle del Bove at <5 km depth. From late June to 1 July, events with M 2.5-3.0 occurred 10-15 km beneath the summit. No significant variations in the volcanic tremor pattern were observed during May or June.

SO2 emissions. (T. Caltabiano and R. Romano.) The average value of SO2 flux in May 1989 was the lowest of the past year, but moderately high values returned in June. SO2 flux was measured 3, ll, 17, and 24 May and 1, 7, 15, 22, and 29 June. Emissions fluctuated in May, with high values on the 3rd and 17th and low values on the 11th and 24th, reaching only 2,500 t/d on the latter date.

Tephra composition. (S. Calvari, M. Coltelli, and M. Pompilio.) January 1989 activity produced hawaiite tephra, with petrography and chemical composition similar to tephra from the previous year. Tephra emitted from Southeast Crater during 1988 had relatively more evolved compositions, but early 1989 tephra was less differentiated than material emitted by the other summit craters.

Geologists observed Strombolian activity at three summit area craters (Bocca Nuova, La Voragine, and Southeast Crater; figure 28) and an incandescent gas vent in Northeast Crater during field work 26 May-1 July. Fresh-looking tephra was abundant W and SW of the summit craters, to 800 m from Bocca Nuova.

Figure 28. Sketch map of Etna's summit region, showing features active during field work 26 May-1 July 1989. Courtesy of C. Oppenheimer and J. Murray.

Bocca Nuova. Vigorous Strombolian activity from two vents on the crater floor, both building cones, was observed 29-30 May, 5, 18, 19, and 30 June, and 1 July. Small explosions and gas bursts averaged ~70/minute. Larger explosions ejected incandescent cowdung bombs, some of which fell 100 m beyond the crater rim. Crater depth was estimated at > 120 m. On 5 June, another glowing vent was visible on or near the flank of the largest active cone. Toward the S end of the crater floor, there was an irregularly shaped area of glowing lava, possibly a ponded flow or passive lava lake. Lava in one of the intracrater vents had a maximum brightness temperature of 1,019°C when measured with a 0.8-1.1 mm infrared thermometer. On 30 June, heat from one of Bocca Nuova's active vents was felt from the observation point on the NW rim.

La Voragine. Strombolian activity occurred from crater floor vents. Loud detonations were heard from the crater rim on 29 May; 17 were counted in 2 minutes. Ejecta were heard falling on the crater floor. During observations the evening of 5 June, four vents were visible, two of which were active. The largest of the 72 explosions counted in a 2-minute period sent bombs to 30 m above the rim. On 1 July, juvenile bombs were ejected by a glowing hornito or small cone on an apparent solidified lava lake. The 0.8-1.1 mm thermometer recorded a peak temperature of 821°C.

Northeast Crater. On 8 June, a vent 2-3 m in diameter was degassing near the middle of the crater floor. A peak brightness temperature of 462°C was recorded by an 8-14 mm infrared thermometer pointed at the vent's inner wall from ~ 50 m distance. Geologists noted that this instrument probably significantly underestimated the gas temperature (see 1 July data). There was no sign of fresh tephra. On 12 June at about 0850, four white "smoke rings" rose slowly from Northeast Crater. On 1 July, glow from the vent was visible during observations in low light shortly after 0600. A 0.8-1.1 mm infrared thermometer yielded a maximum temperature of 644°C, compared to 491° with the 8-14 mm instrument. Eight gas puffs and two more vigorous exhalations were counted in 1 minute.

Geodetic measurements. Very little vertical deformation has occurred since September 1988 (14:01). Slight subsidence was measured E of the summit, with a maximum displacement of 3 cm near the 1987 fissure (relative to a reference station 2.5 km SSW of the summit). A 3.5-cm swelling was measured on the Northeast Rift. A trilateration network on the upper E flank was reoccupied for the first time since June 1988, revealing movements of as much as 4 cm to the NE at stations in the Valle del Leone. The station on Monte Simone (4 km E of the summit) showed an anomalous displacement of 2 cm to the S since September 1987. Stations at Punta Lucia (1.5 km NNW of the summit) and Belvedere (2 km SSE of the summit) were assumed to be stable. Etna's summit elevation was determined at 3,318.4 m, using the base station near Piccolo Rifugio (2,516 m altitude) as a datum. Seismicity was detected optically through the levelling instrument, as in 1988 (14:01). During the early June levelling traverse, 36 shocks were observed, all within 2.2 km of the summit. The largest amplitudes noted were 70 µrad (1.5 km NE of the summit), 111 µrad (near Southeast Crater), and 123 µrad (400 m from Bocca Nuova).

Strong Strombolian activity from La Voragine and Southeast Crater was followed by an explosive eruption from La Voragine during the late morning of 10 September. Tephra rose 600-700 m, and the resulting plume, carried E by the wind, deposited ash and lapilli along a narrow, well-defined, path that extended to the coast. After strong explosive activity and lava fountaining, lava spilled over the rim of Southeast Crater early the next morning, forming fluid, fast-moving flows that reached the Piano del Lago (on the upper S flank). During the nights of 11, 12, and 13 September, four more episodes of strong activity occurred, feeding lava flows that advanced SE, S, and SW; these were also very fluid and descended ~ 2 km to 2,640 m elevation. During this period, with brief exceptions, a small lava lake remained visible within Southeast Crater, and was the location of lava fountains that reached average heights of a few hundred meters.

A violent explosive episode that began at about 0900 on 13 September ejected incandescent pyroclastic material to more than 1 km height. Winds again carried the plume toward the E, leaving a narrow band of scoria, lapilli and ash that reached the coast. Press reports said that tephra fall lasted for ~ 15 minutes and some of the lapilli reached walnut size. No injuries were reported, but numerous cars were damaged, and traffic was slowed by nearly 4 cm of ash on the lower flanks. During the explosive activity, felt seismicity on the upper flanks increased substantially but subsided after a few hours.

The plume from the explosion at 0900 was not evident on weather satellite images, but data returned 4 hours earlier, at 0500, showed a plume 15-20 km wide extending ~ 100 km E from Etna. The plume had dissipated by about 0800. During the day on 14 September, no eruptive episodes occurred, and the magma column remained low.

Preliminary estimates suggest that an area of ~0.5 km2 was covered by ~ 5x105 m3 of lava. No estimates of the volume of pyroclastic material were available at press time. Similar activity occurred at the NE crater in 1977-78.

[Fourteen] strong eruptive episodes that occurred 11-26 September included vigorous explosive activity from Southeast Crater with lava fountains that rose > 1 km. Violent Strombolian activity alternated with periods limited to ash ejection, particularly during the first days of the eruption. Winds initially carried eruptive clouds E and S, then E and NE, with ashfalls reaching Catania (roughly 30 km SSE of the summit). Very fluid lava flows moved S and SSE (Piano del Lago), SE and E (Valle del Bove), and NE (Valle del Leone). The lava flows reached 2,600 m elevation (~ 2 km from the crater) in the Piano del Lago, and the base of the Valle del Bove's W wall at ~2,000 m (3-4 km from the crater).

A team from the Open Univ (P. Francis, C. Oppenheimer, and D. Rothery), the Jet Propulsion Laboratory (L. Glaze and D. Pieri), and IIV (T. Caltabiano) carried out field work 22-29 September. Lava fountaining and Strombolian activity occurred about every 10-12 hours from Southeast Crater. The activity fed many aa flows ~ 100 m long in addition to the more fluid flows that advanced farther downslope. Their detailed chronology of a 24 September Southeast Crater eruptive episode, apparently typical of 23-28 September activity, is shown in table 3.

Table 3. Description of the 24 September 1989 eruptive episode at Etna's Southeast Crater, observed by L. Glaze, C. Oppenheimer, and D. Rothery from 900 m away at the Torre del Filosofo. Temperatures were measured with a Cyclops 33 infrared thermometer (8-14 mm bandpass), but were lower than true values because the 1° field of view always included some air or cold ash.

Time

Description of activity

0940-0955

Gray ash clouds rose 50-100 m above the rim every few minutes. The Central Crater was fuming strongly, but produced no ash.

A fissure developed on the crater's SE flank, and lava fountaining from the fissure fed a flow. The fountain's infrared temperature was 614°C at 1211. By 1224, the fissure had become the source of the main column, with incandescent material rising 200-300 m.

1233

Roaring noise stopped for about 3 seconds, then activity declined and had virtually ceased by 1238.

Geologists from the Ruhr Univ visited Etna 22-30 September. Eruptive episodes that included lava fountains up to 500 m high, ash emission, and lava flows from Southeast Crater and associated fissures, occurred once or twice daily during their first five days. They provided the following chronology of activity observed from Nicolosi, 15 km to the S.

24 September, 0930-1200: Initial small ash puffs from Southeast Crater were followed by high lava fountains. New fissures opened at the base of Southeast Crater, and lava flows moved S, cutting off the road to the Torre del Filosofo (900 m from Southeast Crater).

25 September, 0530-0900: Lava fountains rose 300 m, a 4-km vapor plume was ejected, and lava flowed into the Valle del Bove. Loud rumblings were heard from Nicolosi near the end of the activity. Strong ash emission from Southeast Crater and Bocca Nuova lasted from 0700 to 0900. 1900-2000: Lava fountained to 300 m height from at least four vents in Southeast Crater and a lava flow advanced toward the Valle del Bove.

26 September, 0600-0710: Lava fountains reached 400 m height and powerful ash ejection fed a column that rose 5-6 km above the summit within 30 minutes. Lava flowed into the Valle del Bove. At 0645, a brown, cauliflower-shaped ash cloud rose from the Valle del Bove, suggesting a possible flank outbreak. However, ash emission from that site ceased after a few minutes and no flank eruption was confirmed by other observers. 1210-1215: A series of ash puffs rose from Southeast Crater, but no additional eruption followed. 2000-?: High lava fountains were ejected, lava flowed to the Valle del Bove, and a new vent formed near Southeast Crater.

27 September, about 1200: Ash emission resumed from Southeast Crater. Dark gray ash clouds rose continuously to ~ 100 m above the vent before being carried E by strong winds. Ash emission continued through the evening. After nightfall, a bright glow was visible above the Valle del Bove and small fountains emerged from Southeast Crater once or twice/minute.

R. Romano reports that in the late evening of 27 September (around 2230) two new sub-parallel eruptive fissures opened on the upper E flank at 2,600 and 2,575 m altitude (between Valle del Leone and Valle del Bove, SE of Pizzi Deneri), preceded by a brief seismic crisis. Strombolian activity, violent at times, was initially continuous along the fissures, forming hornitos and scoria ramparts. Lava flows from the upper fissure did not extend beyond (but generally entered) the main lava channel formed by the primary effusive vent at the base of the lower fissure (2575 m altitude). The lava flows, moving generally SE, passed S of Monte Simone, widening at ~ 1,750 m altitude.

During the morning of 28 September an extensive field of NNW-trending non-eruptive fractures formed in the Piano del Lago, propagating in succeeding days to the W and S wall of the Valle del Bove. The fractures also extended downslope, past the edge of Serra del Solfizio, on a NW trend. By the morning of 2 October new fractures had opened to the Zafferana-Rifugio Sapienza road (route 92) ending around 1,500 m elevation, below the effusive vents of 1792. Total length of the fractures was ~6 km.

Ruhr Univ geologists climbed Etna on 28 September. Ash emission from Southeast Crater remained continuous. Around noon, brown ash plumes rose from the vent in pulses every 3-5 seconds, accompanied by block and bomb ejection. No glow was seen. Dense weather clouds obscured visibility during the afternoon, but a distinct increase in noise suggested the onset of Strombolian explosions. As night fell around 1800, weather clouds dissipated and Strombolian bursts were visible every 5-10 seconds, ejecting bombs and spatter to 300 m height. The lava flow in the Valle del Bove apparently emerged from a fissure in its NW part. The non-eruptive fractures that had opened parallel to and ~ 50 m W of the rim of the Valle del Bove during the morning, had vertical displacements that sometimes exceeded 1 m; some were 1.5 m wide and several meters deep. The next day, ash and bomb ejection from Southeast Crater increased considerably at about 0730, with dark gray columns rising 150-200 m despite a very strong W wind. Large bombs often rose higher, falling on the flanks of the cone that was rapidly growing around the vent. Expulsion of ash and blocks occurred every 5-10 seconds, accompanied by hissing and rumbling sounds. No glow was visible. Numerous impact craters as much as 1 m wide and 0.5 m deep, probably produced by the strong 13 September activity (14:08), were found at the site of the 1971 Observatory cone, 500 m SW of the active crater. At 1900, lava fountains were again visible in Southeast Crater, with bombs and spatter sometimes rising 350 m. Lava continued to flow into the Valle del Bove. Similar activity was continuing late 30 September.

R. Romano reports that during the following days a wide lava channel formed, extending E after passing Rocca Musarra to the S. Lava flows branched from this channel, passing a line connecting Monte Calonna and Monte Fontana on 6 October, and reached ~ 1,100 m elevation (~ 6 km from the fissure vents). The lava flows stopped ~ 3 km from the nearest town (Milo). Lava production from the vigorous effusive vent at 2,575 m altitude was very strong during the first days of its activity (30-40 m3/sec), started to diminish beginning 3 October, and stopped completely on 9 October.

Preliminary estimates indicate that a [3] km2 area was covered by lava, and lava volume was [~24] x106 m3 (within the Valle del Bove). No estimates are available for volumes of pyroclastic material and lava ejected by the Southeast Crater. Information remains preliminary and incomplete, and will be revised in coming months with more detailed reports, geophysical and geochemical data, and maps of the lava flows and fractures.

Eruption chronology. "The vigorous Strombolian activity at three summit area craters (Bocca Nuova, La Voragine, and Southeast Crater) observed during May, June, and July (14:07) declined during the first 20 days of August, then progressively resumed in the last 10 days of the month. On 29 and 31 August, after some days of Strombolian activity, La Voragine emitted a 700-m-high tephra column with fallout of lapilli and ash that affected the SE flank to the coast, 19 km from the crater.

"In the first days of September, Strombolian activity also increased at Bocca Nuova and in the morning of 10 September an explosive eruption, identical to those of 29 and 31 August, occurred again at La Voragine. Beginning in the early hours of 11 September, Strombolian activity also resumed at Southeast Crater, then progressively evolved into lava fountaining. Lava spilled over the rim of Southeast Crater, forming two fluid, fast-moving flows that traveled ~2 km SE. Explosive activity and lava spillover then suddenly ceased, indicating lowering of the magma column in the conduit. On the evening of the same day, and 12 more times until 27 September (figure 29), new episodes of vigorous Strombolian activity, evolving into lava fountaining with effusion, were observed at Southeast Crater, each suddenly ceasing and giving rise to slow degassing followed by the resumption of less vigorous Strombolian activity. Lava flows emitted during these 14 effusive episodes were confined to the upper S and SE flank area (Torre del Filosofo-Cisternazza) and descended 2.5-3 km S and SE (figure 30)."

Figure 29. Schematic chronology of Etna's September-October 1989 eruption. Periods of eruptive activity on the NE and SE fractures are indicated by x's.

During fieldwork on the 23rd, geologists observed that the area between Southeast Crater and the TDF was covered by fresh aa, in a flow field that had advanced around the TDF and continued southward. The most distal lobes stopped moving on 22 September.

"Near the end of the lava fountaining phase, in at least six cases (13, 19, 22, 23, 25, and 26 September) a sustained tephra column was formed, with consequent lapilli and ash fallout over the E and SE flank to the coast. On [24] September, two fracture systems, trending [N40°E] and N140°E, opened on the flanks of the newly formed Southeast Crater cone, down to elevations of 2,800 and 2,900 m respectively. An appreciable quantity of lava spilled either over the Southeast Crater rim or from the two newly formed fractures on 25 September and in the following two days. In the morning of 25 September a lava flow from the NE fracture descended 3 km in the Valle del Bove, to 1,900 m (M. Simone area, figure 30). The last episode of lava fountaining and effusion at Southeast Crater occurred the morning of 27 September. Since that afternoon the crater has been characterized by Strombolian activity and ash emission."

On the evening of 27 September, the focus of activity shifted to the NE fracture system. "At about 2200 a small fast-moving flow was emitted from the terminus of the NE fracture, and descended 2 km in the Valle del Bove. The Strombolian activity of Southeast Crater then [temporarily] ceased, and between 2300 and 2330 a series of eruptive vents opened at 2,550 m elevation in the Valle del Leone (upper part of the Valle del Bove), preceded and accompanied by a vigorous seismic swarm. These vents were aligned along a N50°E fracture, on the extension of the fracture cutting the NE flank of the SE cone (figure 30). Lava poured from 3-4 vents, where degassing was minor with ejection of lava fragments to only a few meters height. The various lava tongues converged a short distance from the fracture into a main channel, 10 m wide, where lava advanced rapidly. At dawn on 28 September the flow had descended 5 km in the Valle del Bove, reaching Rocca Musarra at ~ 1,650 m elevation.

"That morning, the SSE fracture (extending N140°E from Southeast Crater) was also seen to have progressed downslope. This newly formed part of the fracture system affected a 100-m-wide sector, extending down 2.5 km (to Cisternazza) and to the upper (W) part of the Valle del Bove. Fracture propagation continued on 29 and 30 September, when the S scarp of the Valle del Bove was affected. On 1 October the fracture, accompanied by marked collapses along the fissure system, crossed the Valle del Bove scarp and propagated farther downslope, following the feeding fissure of the 1792 flank eruption. At 1200 on 2 October the fracture cut route SP92 connecting Zafferana to Rifugio Sapienza. The seismic activity that accompanied the entire fracturing process peaked at that time, with ~40 shocks/hour (figure 31). In the following hours, seismic activity rapidly declined and fracture propagation stopped at ~ 1,500 m elevation, 8 km from Southeast Crater. In the following three days only widening of single fissures was observed, with development of consistent compressive structures (uplift folding, reverse faulting) on both sides of the main extensional (open) fissure."

Figure 31. Daily number of seismic events recorded by station ESP, 1 January-5 October, 1989.

During fieldwork the night of 1 October (by C. Oppenheimer, M. Pompilio, M. Coltelli, and V. Scribano) the flow near the NE fracture's main vent was ~ 8 m wide, and its center was moving at faster than a walking pace. Blocks of lava tossed onto the flow welded instantly but did not sink. A maximum lava surface brightness temperature of 1,039°C was measured by a 0.8-1.1 mm bandpass infrared thermometer (recording for 10 minutes, 3 m from the flow top).

Guy Kieffer notes that the axes of the two fracture systems that opened during the eruption had both been frequently active during the eruptive cycle that began in 1971: the NE axis in 1971, 1978, 1979, and 1986-87; and the SSE axis in 1971, 1978, 1979, 1983, and 1985 (along a southward extension). The NE axis had also been active during the 1811, 1865, and 1928 eruptions, and the SSE axis in 1792, 1908, and 1910. As in the present eruption, fissures had opened from the SSE base of the central cone in 1874, 1911, and 1923 without producing any lava, while lava was emitted from N flank fissures. The 1989 eruption's SSE fracture had opened ~ 45 cm and had a right-lateral displacement of ~ 15 cm (on 7 October) where it crossed route SP92. Kieffer suggested that the SSE fracture was the result of an offset caused by intrusion below Southeast Crater, added to several years of accrued stresses as part of the E flank is forced eastward.

"Lava outpouring from the Valle del Leone fissure continued until 2 October without significant variation in the effusion rate. From 3 October, oscillations, but with a slow progressive decrease, were observed in the rate of lava effusion. During temporary phases of effusion rate increase, eruptive vents were reactivated at higher elevation on the NE effusive fracture. New hornitos were formed, 50 m above those at the 1,550-m vents, accompanied by a series of lava overflows. The effusion rate decreased further on 6 October and lava emission stopped during the night of 8-9 October.

"During the same period, explosive activity continued at Southeast Crater. After the late morning of 28 September ash-laden clouds were emitted from the crater, which also showed Strombolian activity of variable intensity." When observed by C. Oppenheimer on 29 September, discrete eruptions of ash and blocks about every 30 seconds were superimposed on continuous ash emission. Ballistic trajectories of incandescent bombs that rose >400 m above the crater were visible at night.

"Tephra were dispersed eastward by the wind, causing ash showers on Catania and eastern villages, especially on 3 and 4 October. Limited episodes of lava fountaining last occurred during the night of 3-4 October. Strombolian activity stopped during the night of 6 October, whereas discontinuous weak ash emission continued until 9 October. Southeast Crater appeared almost totally obstructed on 11 October. Ash emission and vigorous degassing continued, since 9 October, only at Bocca Nuova.

Lava flows. "Etna's 27 September-9 October flank eruption produced an aa flow field in the Valle del Leone-Valle del Bove ~ 6.5 km long and 5-10 m thick, with a volume of 10-20 x 106 m3. The flows descended from ~2,550 to 1,100 m elevation. The average effusion rate is estimated at 10-20 m3/second, with peak daily average values during the first 4 days of eruption (until 2 October) of between 15 and 25 m3/second. Combinations of the overall emplacement characteristics (e.g. length-average effusion rate, length-volume, width/length-duration) of the flow fields lie within ranges typical for Etna's historic lavas. Velocity and dimension measurements of active overflows at the feeding vent (Valle del Leone, ~ 2,550 m asl) on 2 and 4 October suggest apparent viscosities for the initial lava of the order of 10,000 Pas, for bulk shear rates between 0.01 and 0.1/s. Thermocouple data yielded a surface temperature of 800°C on the overflow (after it had advanced ~ 50-75 m) on 2 October. Preliminary analysis of infrared data obtained on the ground on 5 October suggests a similar temperature for incandescent cracks in an active flow front ~ 6 km from the vent.

Petrology. "The lavas are porphyritic (P.I. = 30-40) alkali basalts with phenocrysts of plagioclase, clinopyroxene, and olivine, and microphenocrysts of Ti-magnetite. There are only minor differences, in size and relative abundance of phenocrysts, between the Southeast Crater flows and the Valle del Leone lavas. Two representative chemical analyses are shown in table 4. The 1989 lavas are slightly more mafic and magnesian than those of either the last Etna eruptions (1986-87 average) or Southeast Crater in 1988.

Civil Protection aspects. "The lava flow emitted from the SE fractures in the Valle del Leone did not present any serious civil protection problem. Flow fronts advanced very slowly at elevations lower than 1,200 m. The flow was confined within the totally uninhabited Valle del Bove and never threatened villages or lifelines. However, the propagation of the SE fracture system from Southeast Crater downslope caused serious concern. Because of strong seismic activity and marked uplift on that sector of the volcano, route SP92 was closed to traffic 24 hours before it was cut by the fracture. The terminus of the fracture was located in a zone that had been affected by flank eruptions in the past (e.g. in 1792). It was at low elevation (~ 1,500 m) and only 5-6 km from villages (Malpasso, Fleri), a distance that could be traveled by a fluid lava flow in a dangerously short time, considering that magma was at the surface in Southeast Crater vent (3,000 m elevation) and hence there was a very high potential load (1,500 m of lava column) on the fracture apex.

"The more probable paths of lava flow in case of effusion from a vent at 1,500 m elevation were estimated on a morphological basis. People living on the potential lava trajectories were alerted to the potential risk and a plan was established for their prompt evacuation.

"Scientific monitoring at the fracture terminus was expanded and reinforced with a mobile seismic network, precision levelling, microgravimetry, geodimeter lines, and soil gas geochemistry. I gas;sampling Visual observation points (24 hours/day) were established near the fracture apex and along the expected flow path, radio linked with the army units responsible for the emergency plans.

"The alert system was maintained until 16 October. Then, with six days having passed since the end of eruptive activity and without any sign of potential volcanic activation of the SE fracture, the eruption was considered concluded.

Seismicity. "Seismic activity has been monitored since early summer 1989, using both the IIV's permanent array (8 stations) and a temporary array operated by the OV (up to 11 three-component telemetered digital stations). The daily frequency histogram is shown in figure 31.

Pre-eruptive period."Five major seismic sequences were recorded from late July to late August; none of the events exceeded M 3.3. Focal depths, calculated with respect to the highest stations (at 2,900 m asl) ranged from ~ 15 km (3 August, 45 events) to

Eruptive period. "On 23-24 September, a swarm of 84 events (M >3.2) was located at ~20 km depth. Three days later, during a period of large fluctuations in tremor amplitude, intense microseismic activity preceded and accompanied the opening of the eruptive fissures on the E flank. Because of its weakness (only 19 events of M>2), this activity was clearly observed only at the nearest station (PDN, ~ 1 km from the new vents).

"More than 1,000 shallow events (depth

Volcanic tremor. "Volcanic tremor was monitored at stations PDN (hourly analysis) and SLN (real-time analysis). A rapid increase in tremor energy was recorded during the morning of 10 September. The level of tremor remained high for four days, with an estimated daily energy release of 1017 ergs (1010 J), then dropped by about an order of magnitude (figure 32). Between 18 September and 6 October, near-periodic (daily/half-daily) oscillations could be distinguished in the tremor amplitude (minor peaks in energy release on 19, 22, 25, and 27 September, and 4 October). With the end of the eruption on 9 October, tremor amplitude dropped to the pre-crisis level. Energy spectra analysis of 5-minute samples from station SLN shows steady peaks in the 1.4-1.9 Hz band, independent of energy level, while spectral displacements to higher frequencies were occasionally observed at station PDN.

Ground deformation - tilt. "Tilt data have been collected at an IIV borehole biaxial tilt station (SPC), located along the SE flank at ~ 1,600 m elevation. Continuously monitored tilt signals indicated a positive variation of the radial component (crater up), clearly exceeding the signal confidence limit, since the beginning of June 1989. The tangential component of the tilt remained practically flat until the beginning of the fracturing phase affecting the SE flank by 2 October. Both radial and tangential tilt underwent a sharp variation by the afternoon of 1 October, suggesting a rapid inflation of the Valle del Bove's S outer flank, where the fracture later propagated. The almost constant ratio between the two tilt components throughout this phase of the deformation process depicts a resultant tilt vector trending approximately N25°E (up) and totaling 14 µrad by the end of the deformation phase.

"Additional tilt data were obtained by two stations (sensor made of a pair of horizontal pendulums) installed by IIV and IPGP. The two automated stations (GIA and TRE) recorded an uplift of the summit area since the first 10 days of June (radial component, figure 33). Marked variations were recorded during the SE fracture propagation, particularly at TRE station, only 200 m from the fracture.

Ground deformation - geodimeter. "A trilateration network on the SE flank area had been previously surveyed in 1987, 1988, and May 1989. Daily EDM measurements were made on some of the network benchmarks since 30 September, before the fracture propagated downslope. Horizontal strain solutions for the main deformation phase that occurred on 2 October indicated an overall areal contraction (~152 ppm) with a minimum extension axis of about -206 ± 80 ppm striking N30°E; maximum shear was 260 ppm. Distance measurements across the main fracture system, at ~ 1,700 m elevation, indicated a local extension of up to ~ 1 m between May 1987 and October 1989.

Ground deformation - levelling. "Precise levelling surveys around the circumference of the volcano and across the new fracture system where it cuts route SP92 were carried out 2-16 October. With respect to September 1988, the N part of the volcano had remained stable, while relative sinking had occurred at the station closest to the central crater (20 mm) and across the fault at Piano Pernicana (25 mm; figure 34). Along route SP92, repeated surveys 6-16 October indicated a relative sinking near the new fracture system of ~ 14 mm.

Figure 34. Top: elevation changes between reoccupations of the level line in September 1988 and on 2-16 October 1989, at about 1,500-2,000 m elevation and 4-10 km from the central crater. Directions from the central crater are shown at top. Bottom: elevation changes between reoccupations of the segment of the level line crossing the SE fracture along route SP92 (figure 30), 6 and 16 October, 1989.

Microgravimetry. "On 3 October the pre-existing microgravimetric network (OV, IIV) was expanded near the fracture cutting route SP92. A new 2-km-long E-W line was established, with eight stations symmetrically arranged across the fracture, each coinciding with a precision levelling benchmark. Gravity measurements were carried out on 3, 4, 6, and 11 October. Differences from 3 to 4 October (figure 35) show a positive variation of the gravity field with a maximum (18 ± 7 microgal) corresponding to the fracture. Subsequent gravity variations remained mostly within the error range. Correction for levelling data does not appreciably modify these results.

Geochemistry - COSPEC data. "A slow increase in SO2 output from the summit craters was observed in August 1989, with a transition from very low values (1,000 t/d) to medium-high values (~ 7,000 t/d). In the first half of September, SO2 flux remained around medium values. Since 14 September, coinciding with the Valle del Leone effusive phase, SO2 emission increased markedly with oscillation around 15,000 t/d, three times the average Etna values. [Measured rates of SO2 emission exceeded 25,000 t/d on one day around 18 September and reached ~ 23,000 t/d on another day near 1 October].

Geochemistry - soil gases. "From 3 to 14 October, CO2 was measured in the soil close to the terminus of the SE fracture. CO2 concentrations were determined at a soil depth of 50 cm by an IR spectrophotometer, and varied from 0 to 1,500 ppmv, with a clear anomalous degassing at the fracture which showed a progressive decrease with time (figure 36). In addition, CO2 concentration in the soil was measured continuously at one of the points. Two sharp peaks were recorded on 3 and 8 October, while later CO2 concentrations remained quite constant (figure 37).

Figure 37. Data from continuous CO2 monitoring of soil near the SE fracture, 3-15 October 1989.

"From 9 to 13 October another soil-gas survey was carried out across the fracture. Data from 140-cm soil depths did not show any anomalous H2 and CO degassing but the data from 60-cm depths were significantly higher than atmospheric: they ranged from 3 to 12 ppmv for H2 and 0.3-58.8 ppmv for CO. The meaning of this difference of gas concentration between 140- and 60-cm-deep holes must be further investigated."

The following, from IIV, summarizes the much milder activity in November.

Summit activity. (S. Calvari, M. Coltelli, and M. Pompilio.) November summit activity was limited to discontinuous tephra emission from Bocca Nuova. Tephra emission episodes were frequent during the first two weeks of the month, often associated with deep explosive activity. Tephra emission became sporadic in the second half of November, although continuous activity was observed on the 19th. No juvenile material was ejected. La Voragine and Southeast Crater remained closed, with only weak fumarolic activity on their floors. Degassing from the vent at the bottom of Northeast Crater continued from previous months.

Seismicity. (E. Privitera, C. Cardaci, O. Cocina, V. Longo, A. Montalto, D. Patané, A. Pellegrino, S. Rapisarda, S. Spampinato, and O. Torrisi.) Seismic activity was very low in November. The three most energetic events following the eruption seismicity occurred 31 Oct at 0752 (M 3.0, 9 km depth, in the S. Alfio area, ~ 13 km E of the summit), 10 November at 0319 (M 3.6, 14 km depth, S of the seismic network near Pantano di Lentini, roughly 50 km S of Etna), and 20 November at 0754 (M 2.4, 15 km depth, in the Sciara del Follone area on the N flank). Recorded tremor remained similar to the pattern observed by the end of the eruption.

Ground deformation. (A. Bonaccorso, O. Campisi, G. Falzone, B. and G. Puglisi, and R. Velardita.) No significant variation has been observed in data from a borehole tilt station (SPC) on the SE flank at ~1,600 m elevation.

SO2 flux. (T. Caltabiano and R. Romano.) SO2 flux stabilized at intermediate values (~ 5,000 t/d) at the end of October, but increased in November, reaching 12,000 t/d on the 17th. Rates then declined, to 2,000 t/d on 28 November. A similar fluctuating pattern was observed during the three months (June-August) preceding the recent eruption, and during the same period in 1988 (not followed by an eruption).

Summit activity. (S. Calvari, M. Coltelli, O. Consoli, M. Pompilio, V. Scribano.) After only fumarolic emissions from Southeast Crater in November (14:11), renewed activity began on 16 December with continuous ejection of reddish cinders. The activity continued for the following two days, becoming weaker on the 18th. A new vent on the crater bottom, observed 21 December from the rim, exhibited strong explosive activity, but without visible tephra ejection. The activity became somewhat more intense during the following days. Only very small cinders (probably juvenile) reached the crater rim. Strong, deep, explosive activity was noted at Bocca Nuova, but there was no evidence of new tephra on the crater rim. La Voragine was obstructed and characterized by weak fumarolic emissions from the crater bottom. Gas emission from a vent on the floor of Northeast crater continued as before, but reddish cinders and gas were emitted on the 16th.

Seismic activity. (S. Privitera, C. Cardaci, O. Cocina, V. Longo, A. Montalto, D. Patane, A. Pellegrino, S. Spampinato.) Seismic activity in December returned to levels similar to those preceding the July-October seismic and eruptive activity (14:7-11). Five events with M>2 were recorded at 5-10 km depths on the Valle del Bove and the volcano's W sector. The most energetic event (M 2.8) occurred 9 December on the E part of the Valle del Bove at 9 km depth. The number and energy of low-frequency events increased, with maximum activity recorded on 16 December. Tremor amplitude also increased and was characterized by energy fluctuations, especially on the 18th and 19th.

Ground deformation. (A. Bonaccorso, O. Campisi, B. and G. Puglisi, R. Velardita.) Preliminary interpretation of the signal at the SPC borehole tilt station on the S flank showed no significant variation during December. Deformation measured on the NE trilateration network was characterized by shear, with almost equal moduli of the main strain axes. Distance measurements across the fracture on the volcano's S side indicate general stability relative to 7 October and 11 November surveys.

Summit crater SO2 flux. (T. Caltabiano, R. Romano.) Samples collected 7, 20, and 29 December revealed that SO2 flux had stabilized below the mean value of 4,000 t/d. A relatively low value (2,000 t/d) was recorded in late November. December 1989 SO2 flux was slightly higher than the December 1988 trend, probably related to a longer-period flux component.

Early January activity. A seismic phase characterized by fluctuations in the mean amplitude of volcanic tremor began on 4 January at 1950 and was recorded at the ESP station of the permanent Mt. Etna seismic network. Iseismicity;amplitude By 5 January at 0120, the amplitude had stabilized at medium-high values. The most energetic phase was recorded between 0730 and 0922 on several seismic stations. During the following hours, tremor amplitude decreased, but sporadic 1-minute fluctuations brought the tremor amplitude to normal levels. The seismic phase was probably accompanied by lava fountaining at Southeast Crater, but bad weather prevented direct observation. The activity deposited lapilli and small scoria (up to 4 cm) on the NW side of the volcano in the Bronte-Randazzo area, with a dispersion axis oriented toward the town of Maletto (15 km NW of the summit crater). Scoria 13 km from the summit crater was 2-3 cm in diameter. Fieldwork in the following days revealed that a small amount of lava had poured over Southeast Crater's rim, probably due to lava fountaining. The 300-m lava flow, ~ 50 m wide and 1-2 m thick, reached the lowermost 1971 crater. As of 10 January, Southeast Crater showed no explosive activity.

Summit activity. (S. Calvari, M. Coltelli, O. Consoli, M. Pompilio and V. Scribano.) Eruptive activity that resumed in December at Southeast Crater continued in January, with explosive episodes of Strombolian activity and lava fountaining 4-5, 12, and 14-15 January. The first episode of Strombolian activity deposited 1 to a few cm of ash on the NW flank. Lava poured from the S crater rim, reaching the 1971 eruptive craters (at least 200 m from the vent). A smaller amount of tephra ejected by similar activity on 12 January was carried a few hundred meters WNW by the wind. Lava spilled over the N crater rim, producing a flow 1.5 km long and 20-30 m wide that traveled NE along the September-October fracture system to ~ 2,700 m altitude. When activity ceased, Southeast Crater was completely obstructed by a solidified crust of lava.

The next day, Strombolian activity gradually increased, reaching maximum intensity on the morning of the 15th. Observations from a helicopter on 16 January revealed a new lava flow 50-100 m wide (apparently erupted 15 January) that had flowed over the E crater rim and traveled about 2.5 km SE down the Valle del Bove, stopping at ~2,000 m altitude (S of Sierra Gianicola Piccola).

Sporadic Strombolian explosions with variable intensities resumed 19 January and continued throughout the month. Bad weather prevented field surveys at the other active summit craters, but observations by helicopter showed degassing at the two central vents (Bocca Nuova and La Voragine) and Northeast Crater.

Seismic activity. (E. Privitera, C. Cardaci, O. Cocina, V. Longo, A. Montalto, D. Patane, A. Pellegrino, and S. Spampinato.) January seismicity increased from previous months. Tremor amplitude fluctuated, with increases on 5, 11-12 and 14-15 January, associated with strong explosive activity and lava emission at Southeast Crater. The number of low-frequency events of M<=1 increased from the single shock recorded in December, often becoming more numerous before and after variations in tremor amplitude. A large increase in the number of low-frequency shocks was recorded 19-20 January, but was not accompanied by variation in the tremor amplitude or an increase in energy release. A sequence of 18 events on the NNW side of the volcano at 10-15 km depth had a large energy release. At least nine events reached or exceeded M 2, with 2 main shocks (at 1200 and 1336 on the 28th) reaching M 2.7.

Ground deformation. (A. Bonaccorso, O. Campisi, G. Falzone, B. Puglisi, and R. Velardita.) January tilt recorded at the SPC and CDV borehole tilt stations on the volcano's S flank showed no significant variation from the previous month. EDM surveys across the fracture on the S side (along SP92) and on the N part of the Etna Sud trilateration network showed variations within instrumental error limits.

Summit crater SO2 flux. (T. Caltabiano and R. Romano.) Samples collected from the summit craters on 8, 12, 16-18, 24, and 31 January showed increased SO2 emission preceding eruptive activity. Emissions rose from ~4,000 t/d on 29 December to ~ 26,000 t/d measured 16 January following Southeast Crater explosive and effusive activity on the 14-15th. Emissions returned to moderate values on 17 January (~5,000 t/d) and remained near that level during measurments on 18, 24, and 31 January.

Summit activity. (S. Calvari, M. Coltelli, O. Consoli, M. Pompilio, and V. Scribano.) February activity was characterized by a single strong eruptive episode at Southeast Crater. Summit-area craters generally remained quiet through the rest of February and March. The 1-2 February eruptive episode was similar to several in January. A gradual increase in Strombolian explosions was followed by lava fountaining, and lava flowed over the crater's E rim for 5 hours beginning at 2200 on 1 February. The flow turned toward the Valle del Bove, advancing to ~ 2,000 m altitude, near the terminus of the mid-January flow. During the morning of 2 February, discontinuous Strombolian activity was followed by ejection of scoria that seldom reached a few tens of meters from the rim. Activity changed at about 1330 to energetic, discontinuous explosions that generated rumbling heard at a considerable distance. Blocks more than a meter across fell within a few hundred meters of the crater; much of the slightly vesicular ash was non-juvenile. Similar activity continued until about midnight. After the eruptive episode, the crater was completely obstructed, without any gas emission, until 27 February, when sporadic ejection of dark tephra began from two vents on the crater floor. February activity at other summit-area craters was limited to vapor emission from floors and walls. Emissions were particularly strong from Northeast Crater, where the active vent's walls were strongly incandescent.

Degassing was continuous at the summit craters in March but was not accompanied by Strombolian activity. Degassing occurred from an elliptical vent on the W floor of La Voragine accompanied by sporadic rumbling. Gas was also emitted from two sites on the SE and NW floor of Bocca Nuova. Weak fumarolic activity, from collapse steps that have formed along concentric fractures in Southeast Crater, was strongest from the center of the crater. Degassing also continued in Northeast Crater. On 29 and 30 March, sporadic tephra ejection and incandescence were observed, apparently from a sudden rise in the magma column.

Seismic activity. (E. Privitera, C. Cardaci, O. Cocina, V. Longo, A. Montaldo, M. Patanè, A. Pellegrino, and S. Spampinato.) Volcanic tremor amplitude began a progressive increase on 1 February at 1239, probably associated with increased Strombolian activity at Southeast Crater. Amplitudes peaked at 1940 that day, and at 0048 the next morning as activity was changing from Strombolian to lava fountaining. Other substantial increases in tremor amplitude occurred at 0600-1100, 1855, and 1935. The first of two sequences of discrete earthquakes on 2 February began at 0352. Eight of the events, centered at ~15 km depth on the volcano's N sector, were larger than M 1, the strongest at M 2.6 between 0424 and 0619. The second series of shocks started at 1321, with the two largest events (M 2.8) at 1322 and 1337. Hypocenters were on the Valle del Bove at <1 km depth. From 3 February until the end of the month, seismic activity was at very low levels, with little variation in tremor amplitude or the number of low-frequency shocks. Nine fracturing events exceeded M 1, with a maximum magnitude of 2.5.

Seismic activity in March was characterized by a significant increase in the number of fracturing events. Swarms on 16 and 18 March totaled 124 shocks (M>=1) and brought the month's recorded earthquakes to 153, ~ 3 times as many as in January and February. The 16 March swarm began at 0530 and continued until 0050 the next day. Of the 107 shocks stronger than M 1, 28 were of M>=2 and three of M>=3. The bulk of the most energetic events originated from the central to W part of the edifice at 10-20 km depth, although one (at 1052) was located just NNW of the central crater at ~5 km depth. The strongest shock of the 18 March sequence, which included 17 events, occurred on the SW flank (a few kilometers S of Monte Nero) at ~10-15 km depth. An M 3.3 earthquake on 22 March at 1159 was ~15 km deep, roughly 6 km SSW of the summit (just S of Monte Vetore). The March seismicity was not accompanied by changes in volcanic tremor amplitude, which remained low throughout the month. The number and amplitude of low-frequency events showed little change after 3 February. A new seismic station (PZF) was installed on the lower NW flank (near Maletto), replacing station RCC, stolen in August 1989. With the new site, IIV's Etna network numbers 8 stations.

Ground deformation. (A. Bonaccorso, O. Campisi, G. Falzone, B. Puglisi, and R. Velardita.) Two tilt stations (SPC and CDV) operated during February, both on the S side of the volcano. Data from station SPC generally remained within resolution limits through February and March. A weak anomaly was recorded on the tangential component 18-20 February, then tangential data returned to the normal range. Radial values from recently installed station CDV remained within resolution limits through February, while tangential data began a (negative) excursion on 18 February that totalled 5 µrad by the end of the month. All instruments from this station were stolen on 1 March. Reoccupation of sites that form a triangle along the fracture zone between 1,800 and 1,500 m altitude on the S-SE flank (between benchmarks Bocche 1792, Serra Pizzuta Calvarina, and Mt. Stempato) did not show significant deformation since the previous measurements on 19 January.

Summit SO2 flux. (T. Caltabiano and R. Romano.) Rates of SO2 emission during Southeast Crater's eruptive episode on 2 February were three times mean values. Measurements 7, 14, and 21 February showed considerable variation. The five March measurements yielded SO2 flux of 2,500-14,000 t/d, increasing at the end of the month.

Summit crater activity. Eruptive activity was at Bocca Nuova and La Voragine, while only degassing was observed at the SE and NE subterminal craters. At the beginning of July, the mild degassing that had characterized the central vents during previous months evolved to Strombolian activity, sporadically ejecting juvenile products that reached the rim of Bocca Nuova. An intense eruptive episode began at Bocca Nuova on 7 August at about 1130, lasting for ~ 40 minutes. Strong Strombolian activity alternated with lava fountaining, producing a thick deposit (~10 cm maximum) of vesiculated scoria and Pele's Hair that accumulated on the N and NW sides of the crater rim. Wind carried lighter tephra 10 km NE, where it reached the villages of Vena and Presa (figure 38). Weak Strombolian activity followed, stopping early the next day. During the same period, La Voragine was limited to moderate Strombolian activity that stopped on 8 August. Increased tremor amplitude was recorded during the night of 7-8 August (see below), then tremor declined to low levels.

Collapse of part of the wall between Bocca Nuova and La Voragine 9-10 August produced a landslide deposit that covered pre-existing vents on Bocca Nuova's floor. This deposit was soon penetrated by explosive activity, which formed two new vents characterized by weak Strombolian activity.

Throughout this period, activity at the SE subterminal crater remained limited to degassing. However, a considerable enlargement of the vent was observed in June, accompanied by strong incandescence of the inner walls. The temperature of the fumarolic gas, measured 8 August, reached 615°C. By the end of August, a larger degassing vent (~ 10 m across) had formed on the crater floor where fumarolic activity had previously been most intense. This vent produced only strong gas emission, without explosive episodes. Activity at the summit craters was limited to degassing of variable intensity in September.

Moderate activity April-June was broken by four seismic sequences that occurred 25 April, 17-18 May, and 1-2 and 30 June (figure 39b). Seismic energy release (figure 39a) was also moderate (maximum M 3.0 on 17 May) and a total of 101 shocks of M >= 1 were recorded. The April-May seismicity mainly affected the W sector of the volcano, with seismic activity moving to the E (Valle del Bove) and NE flanks in June (figure 40). Average focal depths were ~15 km, except for the 1 June sequence, which had a focal zone at a depth of <=10 km (figure 41).

Figure 39. Top a): cumulative seismic energy release at Etna's S flank ESP station (figure 38) in the square-root of ergs (solid line) and radial component of ground tilt at nearby borehole station SPC (dashed line with squares). Bottom b): number of earthquakes (M³1) recorded at ESP station; April-September 1990.

During the next two months, the most significant seismic episodes took place on 3 and 8 July, and 27 August. These sequences plus a general increase in background activity caused a significant change in the slope of the cumulative strain release curve. Energy associated with single events remained moderate, never exceeding M 3.1. The total of 148 events recorded in July decreased to 97 in August. The upper NE flank (10-25 km depth) and the Valle del Bove (6-12 km depth) were the areas most affected.

Another seismic sequence (78 events of M >= 1) occurred on the NW flank on 3 September; the average calculated focal depth was about 24 ± 4 km. Seismic activity then returned to moderate levels for the rest of the month.

Volcanic tremor. During April, May, and the first part of June, volcanic tremor amplitude recorded at a reference station (ESP) on the S flank fluctuated from low to moderate values (7-20 mV/_Hz). Beginning in the second half of June, an amplitude increase was observed (20-30 mV/_Hz) that lasted until 7 August. During the night of 7-8 August, a sudden further increase in tremor amplitude coincided with the violent Strombolian activity from Bocca Nuova (see above). After this episode, tremor amplitude returned to low levels (5-8 mV/_Hz), remaining at similar values until the end of September.

Ground deformation. EDM measurements were performed on two geodimeter networks, on the S and SW flanks. The southern network was measured in June, about a year after the last measurement in May 1989. The area covered by the network includes part of the main fracture system that affected the SE flank during the September-October 1989 eruption (14:8-10). Comparisons between May 1989 and June 1990 data showed significant distance variations, mostly for lines in the higher altitude sector of the network. The resulting deformation pattern was characterized by a significant areal contraction. The deformation ellipse was strongly polarized with the minimum extension axis (contraction) trending approximately N29°E. The southwestern EDM network was reoccupied in July, showing only minor slope distance variations from the previous measurements in June 1989. A weak areal contraction was observed. The calculated deformation ellipse had a minimum extension axis (contraction) striking approximately N7°E.

Tilt data were collected at a biaxial borehole station (SPC) on the S flank, close to the ESP seismic station. Recording was interrupted early April-early June by vandalism. The radial component indicated continuous inflation of the volcanic edifice from the beginning of July until early September, closely paralleling the seismic strain release (figure 39a). During the same period, the tangential component remained nearly flat, showing fluctuations within the confidence limit of about ± 2 µrads.

Bocca Nuova. Intense Strombolian activity occurred from 4 or 5 vents that formed on the crater floor between 10 and 15 November. Ejecta only sporadically reached or went beyond the crater rim; most of the pyroclasts were ejected to moderate height, falling within the crater, where they accumulated with minor intracrater lava flows. The activity stopped abruptly on 24 December (associated with a deformation change; see below), and was succeeded that afternoon by strong continuous ash emission that lasted until the beginning of January. Weak Strombolian activity resumed for a few days in early January, ending by the 12th.

Southeast Crater. Typically mild, sometimes pulsating, gas emission was accompanied in December by weak ash ejection during times of the most intense similar activity at Bocca Nuova. A short period of mild Strombolian activity 4-7 January was the first significant eruptive event since the opening of a vent on the crater floor in April 1990.

La Voragine. Only discontinuous Strombolian activity 4-12 January was reported.

Northeast Crater. Frequent explosions occurred, but did not eject any juvenile tephra. Gas emission became intense in October and November, producing continuous incandescence of the degassing vent's inner walls that persisted through the report period.

Only sporadic summit observations were possible during February. These were limited to Southeast Crater, which was characterized by vapor emission. No information about March summit activity is available.

Seismic activity. Seismicity was moderate during October and November. On 7 December, a sequence of 26 events was recorded in a few hours. All were centered on the N flank at 7-24 km depth. Energy release was low, with no shocks exceeding M 2.2. Seven events on 9 January, at ~9 km depth on the SSW flank, were followed by a second swarm (42 events) on 26 January, which was deeper (14-20 km) and distributed over a wider area on the same part of the volcano. A deformation episode was recorded the same day by a nearby tiltmeter (see below). Another, shallower sequence occurred on the SSW flank on 2 February, in the same area as a sharp deformation episode on 24 December. Instrument problems prevented accurate location of a swarm on 8-9 February. A series of moderate earthquakes, on the N flank at ~ 21 km depth, was recorded 19-20 February. Seismicity declined in March.

Tremor amplitude was low in October, but increased 5-6 November, followed by a period of frequent amplitude fluctuations. On 27 November, tremor was strong enough to be detected for a few hours at seismic stations far from the summit craters. It then declined but remained at slightly elevated amplitude. No change in eruptive activity was reported on 27 November. Tremor continued to fluctuate into early December, then remained at low amplitude through March.

Ground deformation. Tilt data were collected at three borehole biaxial stations (SPC, CDV, and MDZ) on the S and SSW flanks. Summit inflation continued from previous months (at SPC), but slowed in November, and had nearly stopped by December. At nearby CDV, no significant radial tilt was detected, but the tangential component showed inflation toward the SW in October that reversed in November and continued on that trend until 24 December. A sharp deformation event was recorded that day on all three tilt stations, and a sudden change from Strombolian activity to continuous ash emission occurred at Bocca Nuova (see above). The pre-24 December trend then resumed at CDV, continuing through January, while similar behavior was shown by the tangential component at SPC.

Another sharp deformation event was recorded 26 January at station MDZ during a seismic swarm centered nearby (see above); a less intense deformation signal was detected at CDV.

Strong degassing continued .. during fieldwork in June and July. Strombolian activity was reported at a vent in the NE part of Southeast Crater. Small explosions occurred almost continuously, with more powerful blasts ejecting material to the level of the crater rim occurring every 10-15 minutes (in July). Meanwhile, a vent in the center of the crater gently degassed. In June, occasional emissions of small (<20 cm) sublimate-covered lithic blocks and scoria occurred from a 20 x 10 m pit in Northeast Crater. Lava was visible within the vent, which continued to glow through July. The vent widened internally, giving the appearance of a large chamber inclined in the direction of La Voragine. The elliptical vent at La Voragine crater (reopened prior to a 24 May visit; 16:05) showed incandescence in July, but not in June. Degassing continued from numerous fumaroles within the crater. The floor of Bocca Nuova crater was hidden by large quantities of gas in June, but in July two scoria cones were seen gently emitting vapor. At night, a strongly degassing vent on the SE side of the crater emitted tongues of incandescent gas at 15-minute intervals. A fumarole (56°C) was observed on the October 1989 fracture where it crossed the Canalone Della Montagnola at an altitude of ~ 2,200 m.

The following is from Steve Saunders. "A resurvey, in July, of an EDM network (67 lines) on the upper S flank showed a shortening of the majority of the lines (56), suggesting that minor deflation had occurred since the previous survey in July 1990. At that time, length increases along most lines were interpreted as resulting from minor inflation of the upper flanks since November 1989."

Information Contacts:H. Gaudru, EVS, Switzerland; T. De St. Cyr, Fontaines St. Martin, France; S. Saunders, West London Institute of Higher Education; W. McGuire, Cheltenham and Glouster College of Higher Education.

Strombolian activity in Northeast Crater had decreased by mid-July, and was no longer apparent on 23 August. Strombolian activity was observed in Southeast Crater in mid-July. All four summit craters were in a state of almost continuous mild degassing through August.

The following is from a report on activity 10-14 July, by J.P. Kloster in LAVE Bulletin no. 33, p. 4.

A network of curvilinear fissures, up to 2 m wide, covered the N, E, and SE parts of Southeast Crater. In the NW part of the crater, a vent 15 m in diameter emitted puffs of gas roughly every 3 seconds. Every half hour, a more violent explosion ejected lava fragments to 200 m above the vent, covering the walls of the crater and occasionally sending projectiles outside of the crater. The largest projectiles were estimated at around 5 kg, and were fluid enough to deform on impact. Two vents emitted slightly incandescent plumes at night. Each explosion was preceded by ~12 seconds of increased incandescence at the non-exploding vent, and corresponding intensification of glow at the exploding vent. On one occasion, a roughly 100-kg block of lava was ejected to 50 m height.

In the S part of Northeast Crater, a long-persistent vent, 5 m in diameter, emitted puffs of gas. At night, the emission was incandescent to 30 m height, with small lava fragments ejected during the most violent explosions. No projectiles fell beyond the crater rim.

About 70 m below Bocca Nuova crater's S rim, gas was strongly emitted from an E-W fissure, several meters wide and 12 m long, that probably formed in October 1989 (14:10). Explosions were heard emanating from the fissure area, roughly every 4-5 minutes. Night glow was visible at the fissure and at the vents of two coalesced scoria cones in the S part of the crater floor.

Very little activity occurred in La Voragine crater. One 20-m-diameter vent, on a small cinder cone, degassed quietly to several meters, and had night glow. Snow on the cone was largely covered by scoriae, suggesting recent activity.

No Strombolian activity or lava emission were observed during a 23 August visit to the summit by J. Dehn and B. Behncke. The vent in Northeast Crater had strong gas emission, accompanied by loud roaring noises, but no solid material was ejected. A dense continuous gas column was rising from Southeast Crater.

An 8-19 October resurvey of a 46-line EDM network on the upper S flank showed little movement since July, with maximum line-length changes of ~0.0015%. Activity at the summit craters was more energetic than in July but had the same general characteristics. Continued Strombolian activity in the NE sector of Southeast Crater periodically increased in strength, with large incandescent blebs of lava rising tens of meters above the crater rim. The open vent in Northeast Crater was degassing strongly and incandescent at night, while the two vents on the floor of Bocca Nuova had mild to strong Strombolian activity. E of Bocca Nuova, a single vent on the floor of La Voragine displayed weak Strombolian activity and daylight incandescence.

Lava emerged from a fissure at 3,000 m altitude, apparently on the SE flank, for about an hour on 14 December. Light ashfalls occurred on small villages below the fissure. The eruption was preceded by hundreds of small earthquakes.

The following, from R. Romano with additional information from J.C. Tanguy, supersedes the preliminary press report in 16:11. Information from Tanguy about the beginning of the eruption was collected thanks to G. Patanè, S. Imposa, R. Cristofolini, A. & O. Nicoloso, and G. Scarpinati.

A SE-flank fissure eruption began near the base of Southeast Crater early 14 December. Activity ended that day from the initial vents, but fissures propagated downslope where more vigorous lava production began the next morning. The eruption produced a substantial lava field and was continuing in mid-January.

After intense Strombolian activity at ... Bocca Nuova and Southeast Crater, eruptive fissures opened during the early morning of 14 December. These extended ~ 1 km SSE from the base of Southeast Crater (figure 42). Strong harmonic tremor was recorded between 0220 and 0420 by Univ di Catania seismometers. Ejection of lava fragments built modest cones and scoria ramparts along the fracture system, while small lava flows were extruded from some vents. More consistent lava production at the end of the fracture system fed a flow that advanced down the W wall of the Valle del Bove, branching into two lobes. These moved E, but did not exceed 1 km in length, reaching ~2,400 m altitude. When chief guide A. Nicoloso reached the area at about 0800, lava production had nearly stopped, although strong gas emission continued at Bocca Nuova and Southeast Crater. Activity from the fissures ceased completely during the morning. Another modest-sized eruptive fissure, oriented NE-SW, opened at the NE base of Southeast Crater, ejecting hot pyroclastic material.

Figure 42. Topographic map showing the 1989 and 1990 flows, and preliminary locations of the 1991-92 lava, eruptive fissures, and the barrier constructed in Val Calanna. The Piano del Trifoglietto is the broad plain covered by 1991-92 lava in the area of the "1991-92" label. Courtesy of R. Romano, T. Caltabiano, P. Carveni, and M.F. Grasso.

During the night, the NNW-SSE fissures that had been active the previous morning continued to propagate downslope. A second seismic crisis heralded the opening, at about 0245, of two sub-parallel eruptive fissures. These developed along a non-eruptive segment of the 1989 eruption's SE fracture, on the W wall of the Valle del Bove between roughly 2,400 and 2,200 m altitude, a total length of ~ 400 m. Strombolian activity began immediately along the new fissures and lava flowed E from the fissures' ends, extending ~ 1.5 km along the floor of the Valle del Bove by 0900 (observation by G. Scarpinati). The flows reached the base of the Valle del Bove during the afternoon, and advanced on the Piano del Trifoglietto, a plain at ~ 1,500 m altitude. That evening, at 2103, a strong shock was felt near Etna and to ~ 75 km SE (in the Siracusa area). Other isolated shocks and swarms of events with M <4 were recorded, particularly during the first few days of the eruption.

In the days that followed, Strombolian activity, sometimes intense, occurred from several points along the fissures, building small cones and scoria ramparts. Impressive phreatomagmatic explosions, accompanied by loud detonations, were clearly felt in towns at the foot of the volcano, especially during the eruption's first few days. The effusive activity created a system of lava flows, some with fronts hundreds of meters wide, which generally moved east, in the Piano del Trifoglietto.

By 20 December, lava had reached ~ 1,500 m altitude and superposition of lava flows began to be observed. During the evening of 23 December, a very wide lava front reached the Salto della Giumenta (at the head of the Val Calanna, ~ 4.5 km from the vent) and a few flows descended into it the next morning. Lava flows almost completely covered Val Calanna during the succeeding days, destroying orchards and drinking water facilities. On 2 January, a very wide flow front, ~ 10 m thick, had reached 950 m altitude (~ 5.5 km from the vent) and was advancing slowly. Construction began that day on a containment barrier along the E side of Val Calanna.

An extensive lava field had formed in the Piano del Trifoglietto, with individual lobes frequently superposing and combining. Overflows began from the N part of the lava field about 2 January, forming a separate flow around the N side of Monte Calanna on 5 January and rejoining the stagnant lava front in Val Calanna on the 7th. Flow fronts in Val Calanna had stopped by the morning of 9 January, while active superposed lobes were noted on the lava that had flowed N of Monte Calanna. The most advanced front was at ~ 1,100 m altitude and was tending to move E. The extensive (1-km-wide) main lava field fed numerous breakouts or ephemeral vents, from which modest flows advanced over earlier lava. The main lava channel, originating around 2,200 m altitude, was being vigorously fed and at times was tubed over.

By 14 January, Strombolian activity from the fissure vents had declined notably and explosions were no longer audible. Effusive activity was concentrated at a single vent, feeding a lava channel that subdivided into several flows at ~ 2,000 m altitude (at the base of the Valle del Bove's W wall). These moved onto the lava field formed earlier in the eruption but did not extend beyond 1,550 m altitude. The area covered by new lava had not grown since 9 January, but numerous ephemeral flows appeared on its surface. The containment barrier at the end of Val Calanna had not been tested as of 14 January, since the nearest flow had stopped ~150 m away (~6 km from the vent). As of 21 January the eruption was continuing, although apparently at a reduced rate.

Degassing from the summit craters has continued since the beginning of the eruption. Activity was sometimes intense, but ash was rarely mixed with the gas. Strombolian activity that was vigorous at times continued from various vents at the bottom of Bocca Nuova.

Romano noted that the activity has the characteristics of a classic "slow eruption" (Romano and Sturiale, 1982) and is very similar to the 1819 eruption that occurred in the same area of the Valle del Bove.

Preliminary estimates indicated that ~40 x 106 m3 of lava had been ejected as of 9 January, with an effusion rate of around 15-18 m3/s. Measurements of the effusion rate on 11 January yielded a value of around 9 m3/s from the lava channel at 2,000 m altitude.

Lava production continues from SE-flank fissure but explosive activity declines

The following is from R. Romano. The SE-flank fissure eruption that began on 14 December was continuing as of early February. No notable decrease in the rate of lava effusion has been observed. Explosive activity along the eruptive fissure, which has been highly variable in intensity and duration, has mainly declined to sporadic ejections of lava fragments, limited expulsion of dark ash, and rare phreatomagmatic explosions. Strong gas emission was generally evident. Seismic activity has declined to isolated weak events.

The lava field in the SE flank's Valle del Bove has grown considerably, reaching a maximum width of 1.5 km in a few places, as flows from the N (dominantly) and S merged. Most of the lava was carried through a complex system of tubes originating at the main effusive vent, and emerged onto the lava field surface through numerous ephemeral secondary vents. These varied daily in number and position, and were concentrated in the middle of the lava field (around 1,550 m asl), but some have recently appeared at ~ 1,450 m altitude (at the base of Monte Zoccolaro, just before the break in slope at the Salto della Giumenta). Lava flows from the secondary vents generally advanced along the Salto della Giumenta, and were sometimes relatively continuous. In early February one of these reached, but did not surpass, the front of the eruption's longest flow, at 1,000 m elevation (in Val Calanna, ~6 km from the active fissure). Preliminary estimates indicate that > 6 km2 has been covered by ~55-60 x 106 m3 of new lava, an average volume for Etna's "slow eruption" type (Romano and Sturiale, 1982).

The following is from a report by the Gruppo Nazionale per la Vulcanologia (GNV) summarizing Etna's 1991-92 eruption.

1. Introduction and Civil Protection problems. After 23 months of quiet, and heralded by ground deformation and a short seismic swarm, effusive activity resumed at Etna early 14 December. The eruptive vent opened at 2,200 m elevation on the W wall of the Valle del Bove, along a SE-flank fracture that formed during the 1989 eruption.

Since the eruption's onset, the GNV, in cooperation with Civil Protection authorities, has reinforced the scientific monitoring of Etna. Attention was focused on both the advance of the lava flow and on the possibility of downslope migration of the eruptive vent along the 1989 fracture system. The progress of the lava flow has been carefully followed by daily field inspections and helicopter overflights.

Because of its slow rate of advance, the lava did not threaten lives, but had the potential for severe property destruction. The water supply system for Zafferana (in Val Calanna; figure 43) was destroyed in the first two weeks of the eruption ($2.5 million damage). On 1 January, when the lava front was only 2 km from Zafferana, the Minister for Civil Protection, at the suggestion of the volcanologists, ordered the building of an earthen barrier to protect the village. The barrier was erected at the E end of Val Calanna, where the valley narrows into a deeply eroded canyon. The barrier was conceived to prevent or delay the flow's advance, not to divert it, by creating a morphological obstacle that would favor flow overlapping and lateral expansion of the lava in the large Val Calanna basin.

Figure 43. Topographic sketch map showing Etna's 1989 and 1990 lava flows, with preliminary locations of the 1991-92 lava, eruptive fissures, and the barrier constructed in Val Calanna. The area covered by lava since 14 January is shown in a separate pattern. The GNV report, received near press time, included a map that differed somewhat in detail from this map, which was prepared by R. Romano, T. Caltabiano, P. Carveni, M.F. Grasso, and C. Monaco. See pg.4 of Barberi et al., 1990 for a map of the 1989 lava flows, fissures, and monitoring network.

The barrier, erected by specialized Army and Fire Brigade personnel in 10 days of non-stop work, is ~ 250 m long and ~ 20 m higher than the adjacent Val Calanna floor. It was built by diking the valley bottom in front of the advancing lava and accumulating loose material (earth, scoria, and lava fragments) on a small natural scarp. On 7 January, the lava front approached to a few tens of meters from the barrier, then stopped because of a sudden drop in feeding caused by a huge lava overflow from the main channel several kilometers upslope.

A decrease in the effusion rate has been observed since mid-January. There is therefore little chance of further advance of the front, as the flow seems to have reached its natural maximum length. The eruptive fracture is being carefully monitored (seismicity, ground deformation, geoelectrics, gravimetry, and gas geochemistry) to detect early symptoms of a possible dangerous downslope migration of the vent along the 1989 fracture, which continues along the present fracture's SE trend. Preparedness plans were implemented in case of lava emission from the fracture's lower end.

Many scientists and technicians, the majority of whom are from IIV and the Istituto per la Geochimica dei Fluidi, Palermo (IGF) and are coordinated by GNV, are collecting information on the geological, petrological, geochemical, and geophysical aspects of the eruption.

2. Eruption chronology. On 14 December at about 0200, a seismic swarm (see Seismicity section below) indicated the opening of two radial fractures trending NE and SSE from Southeast Crater. Very soon, ash and bombs formed small scoria ramparts along the NE fracture, where brief activity was confined to the base of Southeast Crater. Meanwhile, a SSE-trending fracture extended ~ 1.3 km from the base of the crater (at ~3,000 m asl) to 2,700 m altitude.

Lava fountaining up to 300 m high from the uppermost section of the SSE fracture continued until about 0600, producing scoria ramparts 10 m high. Two thin (~ 1 m thick) lava flows from the fracture moved E. The N flow, from the highest part of the fracture, stopped at 2,750 m altitude, while the other, starting at 2,850 m elevation, reached the rim of the Valle del Bove (in the Belvedere area), pouring downvalley to ~ 2,500 m asl. At noon, the lava flows stopped, while the W vent of the central crater (Bocca Nuova) was the source of intense Strombolian activity.

The SSE fracture system continued to propagate downslope, crossing the rim of the Valle del Bove in the late evening. During the night of 14-15 December, lava emerged from the lowest segment of the fracture cutting the W flank of the Valle del Bove, reaching 2,400 m altitude (E of Cisternazza). Degassing and Strombolian activity built small scoria cones. Two lava flows advanced downslope from the base of the lower scoria cone at an estimated initial velocity of 15 m/s, which dramatically decreased when they reached the floor of the Valle del Bove.

The SSE fractures formed a system 3 km long and 350-500 m wide that has not propagated since 15 December. Between Southeast Crater and Cisternazza, the fracture field includes the 1989 fractures, which were reactivated with 30-50-cm offsets. The most evident offsets were down to the E, with right-lateral extensional movements. Numerous pit craters, <1 m in diameter, formed along the fractures.

Lava flows have been spreading down the Valle del Bove into the Piano del Trifoglietto, advancing a few hundred meters/day since 15 December. The high initial outflow rates peaked during the last week of 1991 and the first few days of 1992, and decreased after the second week in January. Strombolian activity at the vent in the upper part of the fracture has gradually diminished.

Lava flows were confined to the Valle del Bove until 24 December, when the most advanced front extended beyond the steep slope of the Salto della Giumenta (1,300-1,400 m altitude), accumulating on the floor of Val Calanna. Since then, many ephemeral vents and lava tubes have formed in the area N of Monte Zoccolaro, probably because of variations in the eruption rate. These widened the lava field in the area, and decreased feeding for flows moving into Val Calanna. However, by the end of December, lava flows expanded further in Val Calanna, moving E and threatening the village of Zafferana Etnea, ~2 km E of the most advanced flow front. This front stopped on 3 January, on the same day that a flow from the Valle del Bove moved N of Monte Calanna, later turning back southward and rejoining lava that had already stopped in Val Calanna. Since 9 January, lava flows in Val Calanna have not extended farther downslope, but have piled up a thick sequence of lobes.

Lava outflow from the vent continued at a more or less constant rate, producing a lava field in the Valle del Bove that consisted of a complex network of tubes and braiding, superposing flows, with a continuously changing system of overflows and ephemeral vents.

3. Lava flow measurements. An estimate of lava channel dimensions, flow velocity, and related rheological parameters was carried out where the flow enters the Valle del Bove. Flow velocities ranging from 0.4-1 m/s were observed 3-7 January in a single flow channel (10 m wide, ~ 2.5 m deep) at 1,800 m altitude, ~ 600 m from the vent. From these values, a flow rate of 8-25 m3/s and viscosities ranging from 70-180 Pas were calculated. Direct temperature measurements at several points on the flow surface with an Al/Ni thermocouple and a 2-color pyrometer (HOTSHOT) yielded values of 850-1,080°C.

4. Petrography and chemistry. Systematic lava sampling was carried out at the flow fronts and near the vents. All of the samples were porphyritic (P.I.»25-35%) and of hawaiitic composition, differing from the 1989 lavas, which fall within the alkali basalt field. Paragenesis is typical of Etna's lavas, with phenocrysts (maximum dimension, 3 mm) of plagioclase, clinopyroxene, and olivine, with Ti-magnitite microphenocrysts. The interstitial to hyalopitic groundmass showed microlites of the same minerals.

5. Seismicity. On 14 December at 0245, a seismic swarm occurred in the summit area (figure 44), related to the opening of upper SE-flank eruptive fractures. About 270 earthquakes were recorded, with a maximum local magnitude of 3. A drastic reduction in the seismic rate was observed from 0046 on 15 December, with only four events recorded until the main shock (Md 3.6) of a new sequence occurred at 2100. The seismic rate remained quite high until 0029 on 17 December, declining gradually thereafter.

Figure 44. Daily number of recorded earthquakes and cumulative strain release (top), with amplitude (middle) and dominant frequency peaks of volcanic tremor (bottom) at Etna, 1 December 1991-mid-January 1992. Arrows mark the eruption's onset. Courtesy of the Gruppo Nazionale per la Vulcanologia.

At least three different focal zones were recognized. On 14 December, one was located NE of the summit and a second in the Valle del Bove. The third, SW of the summit, was active on 15 December. All three focal zones were confined to <3 km depth. Three waveform types were recognized, ranging from low-to-high frequency.

As the seismic swarm began on 14 December, volcanic tremor amplitude increased sharply. Maximum amplitude was reached on 21 December, followed by a gradually decreasing trend. As the tremor amplitude increased, the frequency pattern of its dominant spectral peaks changed, increasing within a less-consistent frequency trend. Seismicity rapidly declined and remained at low levels despite the ongoing eruption.

6. Ground deformation. EDM measurements and continuously recording shallow-borehole tiltmeters have been used for several years to monitor ground deformation at Etna. The tilt network has recently grown to 9 flank stations. A new tilt station (CDV) established on the NE side of the fracture in early 1990 showed a steady radial-component increase in early March 1991 after a sharp deformation event at the end of 1990 (figure 45), suggesting that pressure was building into the main central conduit. Maximum inflation was reached by October 1991, followed by a partial decrease in radial tilt, tentatively related to magma intrusion into the already opened S branch of the 1989 fracture system, perhaps releasing pressure in the central conduit.

Figure 45. Radial and tangential components measured by the CDV borehole tilt station on the NE side of Etna's 1989 fracture, 1 July—mid-January 1992. The signal has been filtered for daily and seasonal thermoelastic noise. Arrows mark the eruption's onset. Courtesy of the Gruppo Nazionale per la Vulcanologia.

The eruption's onset was clearly detected by all flank tilt stations, despite their distance from the eruption site. The signals clearly record deformation events closely associated in time with seismic swarms on the W flank (before the eruption began) and on the summit and SW sector (after eruption onset). The second swarm heralded the opening of the most active vent on the W wall of the Valle del Bove.

S-flank EDM measurements detected only minor deformation, in the zone affected by the 1989 fracture. Lines crossing the fracture trend showed brief extensions in January 1992.

The levelling route established in 1989 across the SE fracture was reoccupied 18-19 December 1991. A minor general decline had occurred since the previous survey (October 1990), with a maximum (-10 mm) at a benchmark near the fracture.

7. Gravity changes. Microgravity measurements have been carried out on Etna since 1986, using a network covering a wide area between 1,000 and 1,900 m asl. A reference station is located ~ 20 km NE of the central crater. Five new surveys were made across the 1989 fissure zone during the eruption (15 & 18 December 1991, and 9, 13, and 18 January 1992). Between 21 November and 15 December, the minimum value of gravity variations was about -20 mGal, E of the fracture zone. On 9 January, the gravity variations inverted to a maximum of about +15 mGal. Amplitude increased and anomaly extension was reduced on 13 January, and on 18 January gravity variations were similar to those 9 days earlier. Assuming that height changes were negligible, a change in mass of ~2 x 106 tons (~2 x 107 m3 volume), for a density contrast of 0.1 g/cm3 was postulated. However, if gravity changes were attributed to magma movement, a density contrast of 0.6 g/cm3 between magma and country rock could be assumed and magma displacement would be ~ 3 x 106 m3.

8. Magnetic observations. A 447-point magnetic surveillance array was spaced at 5-m intervals near the fracture that cut route SP92 in 1989. Measurements of total magnetic field intensity (B) have been carried out at least every 3 months since October 1989. Significant long-term magnetic variations were not observed between February 1991 and January 1992, although the amplitude of variations seems to have increased since the beginning of the eruption.

9. Self-potential. A program of self-potential measurements along an 1.32-km E-W profile crossing the SE fracture system (along route SP92 at ~ 1,600 m altitude) began on 25 October 1989. Two large positive anomalies were consistently present during measurements on 5 and 17 January, and 9, 18, and 19 February 1992. The strongest was centered above the fracture system, the second was displaced to the W. Only the 5 January profile hints at the presence of a third positive anomaly, on its extreme E end. The persistent post-1989 SP anomalies could be related to a magmatic intrusion, causing electrical charge polarizations inside the overlying water-saturated rocks. A recent additional intrusion was very likely to have caused the large increase in amplitude and width of the SP anomaly centered above the fracture system, detected on the E side of the profile on 5 January 1992.

10. COSPEC measurements of SO2 flux. The SO2 flux from Etna during the eruption has been characterized by fairly high values, averaging ~ 10,000 t/d, ~ 3 times the mean pre-eruptive rate. Individual measurements varied between ~6,000 and 15,000 t/d.

11. Soil gases. Lines perpendicular to the 1989 fracture, at ~ 1,600 m altitude, have been monitored for CO2 flux. A sharp increase in CO2 output was recorded in September 1991, about 3 months before the eruption began (figure 46). Measurements have been more frequent since 17 December, but no significant variation in CO2 emission has been observed. Samples of soil gases collected at 50 cm depth showed a general decrease in He and CO2 contents since the beginning of January. Soil degassing at two anomalous exhalation areas, on the lower SW and E flanks at ~ 600 m altitude, dropped just before (SW flank—Paternò) and immediately after (E flank—Zafferana) the beginning of the eruption, and remained at low levels. A significant radon anomaly was recorded 26-28 January along the 1989 fracture, but CO2 and radon monitoring have been hampered by snow.

Figure 46. CO2 concentrations measured along Etna's 1989 fracture, late 1990-early 1992, showing a strong increase about 3 months before the December 1991 eruption. Courtesy of the Gruppo Nazionale per la Vulcanologia.

The following, from R. Romano, describes activity in February and early March.

The SE-flank fissure eruption was continuing in early March, but was less vigorous than in previous months. An area of ~ 7 km2 has been covered by around 60 x 106 m3 of lava, with an average effusion rate of 8 m3/s. The size of the lava field (figure 43) has not increased since it reached a maximum width of 1.7 km in mid-February.

Lava from fissure vents at ~ 2,100 m asl flowed in an open channel to 1,850 m altitude, then advanced through tubes. Flowing lava was visible in the upper few kilometers of the tubes through numerous skylights. Lava emerged from the tube system through as many as seven ephemeral vents on the edge of the Salto della Giumenta (at the head of the Val Calanna, ~ 4.5 km from the eruptive fissure). These fed a complex network of flows in the Salto della Giumenta that were generally short and not very vigorous. None extended beyond the eruption's longest flow, which had reached 6.5 km from the eruptive fissure (1,000 m asl) before stopping in early January. Ephemeral vent activity upslope (within the Valle del Bove) ceased by the end of February. Lava production from fissure vents at 2,150 m altitude has gradually declined and explosive activity has stopped. Degassing along the section of the fissure between 2,300 and 2,200 m altitude was also gradually decreasing.

Small vents were active at the bottom of both central craters. Activity at the west crater (Bocca Nuova) was generally limited to gas emission, but significant ash expulsions were observed during the first few days in March. High-temperature gases emerged from the E crater (La Voragine). Collapse within Northeast Crater, probably between 26 and 27 February, was associated with coarse ashfalls on the upper NE flank (at Piano Provenzana and Piano Pernicana). After the collapse, a new pit crater ~ 50 m in diameter occupied the site of Northeast Crater's former vent. Activity from Southeast Crater was limited to gas emission from a modest-sized vent.

Seismic activity was characterized by low-intensity swarms. A few shocks were felt in mid-February ~ 12 km SE of the summit (in the Zafferana area).

Lava production continues from SE-flank vent; town threatened by lava flow

Most of the following is from R. Romano.

The fissure eruption ... was continuing in mid-April. Lava production from the main vent at 2,100 m altitude (in the W wall of the Valle del Bove) has remained almost constant, accompanied by degassing of varying intensity. By mid-April, 85 x 106 m3 of lava that had emerged at an average rate of 8 m3/s was estimated to cover an area of >7 km2.

From March through mid-April, lava from the main vent immediately entered a complex tube system, reappearing through ephemeral vents ~ 5 km downslope, below the Valle del Bove. The vents formed on the edge of the Salto della Giumenta, at the head of the Val Calanna a few kilometers from Zafferana Etnea, population ~ 7,000 (see figure 43). No ephemeral vents or lava overflows have been observed since the end of February in the wide lava field that had developed in the S part of the Valle del Bove.

On 14 March, lava reached the base of the barrier constructed in early January at the bottom of the Val Calanna. During the second half of March and the first few days of April, lava from the ephemeral vents gradually filled the area from the base of the Salto della Giumenta to the barrier. On 3 April, vents began to form within the Salto della Giumenta, feeding flows inside the Val Calanna. By the morning of 7 April, lava was only a few meters from the top of the barrier, with an active front along the barrier's entire length. That afternoon, lava flowed around the S side of the barrier and began to advance along the access road in the Val Calanna. The next afternoon, lava spilled over the central part of the barrier, and began to move down the gorge of the neighboring Portella Calanna valley. Lava advanced rapidly, aided by the steep slope, covering 1 km in 5 days. At least six earth barriers were built to contain the lava, but none were successful. By 14 April, lava was overflowing the last barrier, at ~ 780 m elevation, 1.5 km from the inhabited center of Zafferana Etnea and 7.5 km from the main vent.

Efforts were also made to slow or halt the advancing lava by disrupting the feeder tube system. Experiments with directed explosives, designed to blast holes in the lava field and encourage lava breakouts, began on 13 April in the upper Valle del Bove and Val Calanna. Lava destroyed two isolated houses above Zafferana on 14 April and covered nearby orchards, but the lobe threatening the town had virtually stopped by the next day and evacuation plans were postponed. New lava approached Zafferana over earlier flows in the succeeding days, and was again within a kilometer of the town by 20 April.

Degassing from the summit craters has gradually decreased following the collapse episode in Northeast Crater at the end of February (17:02). Additional Northeast Crater collapses 21-29 March triggered ash ejections accompanied by distinctive seismicity. Moderate gas emission from the summit vents observed by B. Behncke on 21 March was punctuated around 0900 by several low-energy ash emissions from Northeast Crater. At 1230 on 24 March, a dark-gray, convoluted ash plume rose to ~ 1 km above the summit within ~ 5 minutes, and also appeared to spread laterally at its base. The plume gradually dissipated during the following 30 minutes, depositing tephra to the E.

Lava has emerged from a SE-flank fissure in the W wall of the Valle del Bove since 15 December, covering an estimated 7.3 km2 with ~ 100 x 106 m3 of lava. A well-developed tube system carried lava downslope, threatening the town of Zafferana Etnea and prompting attempts at lava diversion (figure 47). The lava production rate, as observed through numerous skylights along the main lava tube, has remained relatively constant, but distal flow fronts advanced at varying rates. The apparent intensity of gas emission from the eruptive fissure changed with weather conditions. During the last 10 days of April, fumarolic activity was observed in the W wall of the Valle del Bove, extending upslope from the eruptive fissure along its NNW trend. This zone was active on 14 December during the initial phase of the eruption.

Figure 47. Topographic sketch map showing Etna's 1989-92 lava flows, with preliminary locations of the 1991-92 eruptive fissures, and the barrier constructed in January in Val Calanna. Areas covered by lava since 14 January and 10 March are shown in separate patterns. Asterisks mark sites of lava diversion experiments in April and May. Courtesy of R. Romano, T. Caltabiano, P. Carveni, and M.F. Grasso.

Lava overwhelmed a series of barriers in early April, and advanced 1 km down a gorge (within the Valle di Portella Calanna) toward Zafferana during the second week in April. This flow stopped on 15 April at 750 m elevation, roughly 1.5 km from the inhabited center of Zafferana. Numerous ephemeral vents began to form below 1,000 m elevation on 19 April (on the E edge of Val Calanna, in which a barrier had been built in early January). Flows from these vents covered lava from previous days along the gorge below Portella Calanna. The longest stopped during the evening of 25 April at 755 m altitude, ~ 7.5 km from the eruptive fissure and 1.5 km from the center of Zafferana. Lava flows originated from a large ephemeral vent at the head of Val Calanna in the beginning of May, passing Portella Calanna atop previous flows on 6 May and reaching 850 m asl that evening.

Ephemeral vents also developed upslope, within the wide lava field that had formed in the S part of the Valle del Bove during previous months. The first formed around 1,900 m altitude (at Monte del Rifugio Menza) on 22 April, and a second occurred near the center of the lava field, at around 1,550 m elevation (near Poggio Canfareddi) on 25 April. Flows from these vents were not very substantial and were no longer active a few days later. On 5 May, only a modest active vent at the N edge of the lava field (around 1,600 m elevation) was observed.

Experiments with the use of explosives, cement blocks, and, more recently, lava blocks continued at skylights in the main lava tube (in the upper Valle del Bove, at around 2,100 m altitude, on 17, 21, and 29 April, and 4 May) and at ephemeral vents (near Portella Calanna on 15 April and in Val Calanna on 6 May). These were designed to cause lava overflows and thus reduce the amount of lava carried in tubes toward inhabited areas. As of early May, it was difficult to evaluate whether these experiments had favorably affected the course of the eruption. On 4 May an overflow began from a skylight in the main lava tube at around 2,100 m altitude, where blocks of cement had been dropped and explosives detonated on previous days. The modest overflow moved over the lava field that had formed in the preceding months. However, during this time, numerous ephemeral vents, varying daily in number and location, remained active above Zafferana at the head of Val Calanna (in the Salto della Giumenta).

Slow, weak degassing continued through early May from the summit craters. Weak ash ejections, caused by internal collapse, were observed only from Northeast Crater, on 22 April. Early April-early May seismic activity was much reduced from previous months.

The following is from R. Romano. Lava production from the fissure ... was continuing without noticeable variation in mid-June. Gas emission, from four explosion vents between 2,335 and 2,215 m elevation, has diminished along the upper part of the fissure. The main lava channel has roofed over, but lava was visible through a skylight beginning at 2,205 m elevation, where the effusion rate was estimated at 6-8 m3/s and the flow velocity at ~ 1 m/s on 7 and 13 June. Three more skylights were open along the main channel to 2,020 m asl. An overflow occurred on 12 June from one of the skylights, at 2,075 m altitude, but lava advanced only a few meters before returning to the main channel. This overflow was still active the next day. Ephemeral vents from the main tube remained active through the end of May: in the Valle del Bove; below the Valle del Bove in Val Calanna; and near the distal end of the flow field, along a deep gully under Portella Calanna (figure 48). Lava flows emerged more or less continuously from the latter vents, but did not descend below 800 m altitude. The total volume of lava produced by the eruption is estimated at 150 x 106 m3.

Figure 48. Status of activity within Etna's flow field on 18 May 1992, after 153 days of activity. Modified by Hughes and Bulmer from map by Romano in 17:4. Contour interval, 100 m.

Lava diversion. An earthen barrier built in a valley above the town of Zafferana Etnea in early January was breached by lava on 7 April. Lava overran a series of additional barriers the following week but stopped before reaching the town. Subsequent hazards efforts focused on reducing the lava supply to the end of the flow, by obstructing the main lava tube near the vent and disrupting lava production at ephemeral vents (17:3-4).

F. Barberi and L. Villari report successful lava diversion from the main tube, at a site 500 m downslope from the primary eruptive vent. In this area, at ~ 2,000 m elevation on the W wall of the Valle del Bove, lava was carried through a single tube locally broken by skylights. On 27 May, about 2/3 of the tube's lava was diverted into an artificially excavated channel by blasting through the 2-3-m-thick wall of the right levee. Two days later, bulldozers obstructed the natural channel by pushing large blocks of lava into it. By 1815 that day, all of the lava output (~30 m3/s) was flowing into the artificial channel. In effect, the diversion returned the active flow front to its position a few days after the onset of the eruption. Lava was moving downslope along the same path as the earlier main flow, but was > 6 km upslope from its previously most advanced front.

Flows generated by lava diversion efforts. R. Romano reports that as of 13 June, a vent remained active at the site of the first lava diversion. Although the vent has been shrinking, it continued to feed a flow that has advanced over lava from previous months, forming tubes and various ephemeral vents, many of which were near the S wall of the Valle del Bove. The ephemeral vents produced two lava flows, one near the S wall of the Valle del Bove at around 1,700 m elevation, the other in a more central position, at ~ 1,800 m asl on the main lava field. The lava flows that formed after the first diversion advanced more than a kilometer over the center of the lava field. Flows that followed the second diversion remained predominantly near the S wall of the Valle del Bove, passing and encircling a site at 1,575 m asl (Poggio Canfareddi), 2 km from their point of origin, on 3 June. This lava front stopped advancing on 5 June and several superposing lobes began to develop.

Seismicity and summit activity. Weak seismic activity began on 29 May, followed by an increase in volcanic tremor on 31 May that continued until the next day. Ash emissions, sometimes voluminous, occurred from the central craters at irregular intervals on 31 May and 1 June, first from the W vent (Bocca Nuova) then from the E vent (La Voragine). Only weak degassing preceded the ash ejection, but gas emission became more consistent beginning 2 June. COSPEC measurements yielded SO2 flux values of ~ 10,000 t/d. Flashes from the summit craters were observed during the evening of 7 June from the W flank. Fieldwork on 12 June revealed that Northeast Crater was obstructed, with only fumarolic activity along the walls.

EDM data. S. Saunders reports that four lines of an EDM network on the upper S flank were remeasured on 7 May, showing a 138-ppm contraction that was interpreted as deflation during the eruption. Between July and October 1991, total extensional strain along these lines was 88 ppm, indicating pre-eruption inflation. Strain along these lines has returned to near pre-eruption levels.

Landsat Thematic Mapper data. The following is from D. Rothery. "The 1991-92 sustained lava eruption of Etna provides an opportunity to study lava flow development by remote sensing. The first cloud-free Landsat Thematic Mapper (TM) image of the eruption was recorded on 2 January at approximately 1000 (figure 49). Landsat repeats its coverage on a 16-day cycle; the next cloud-free acquisition was on 22 March and we are still awaiting receipt of those data. By manipulating radiance measurements in two wavebands, we hope to be able to constrain the surface temperature distribution of this flow along its length. The most noteworthy aspects of the 2 January data are: 1) There is a narrow 700-m length near the source that is radiant in TM band 4 (0.76-0.90 mm wavelength). As far as we know, this is the first time that thermal radiance in TM band 4 has been reported over a volcano. Field observations (A. Borgia) on 2 and 3 January show that this feature corresponds to a 10-15-m-wide open channel at the source of the flow. 2) The entire 6.5-km-long active flow is radiant in TM band 7 (2.08-2.35 mm wavelength). At least some of the areas that are also radiant in band 5 (1.55-1.75 mm) occur when the flow spills down a steep slope, breaking apart the raft of blocks and crust that otherwise blanket the underlying lava at near-magmatic temperatures."

Figure 49. Extracts of Landsat TMr images of Etna, 2 January 1992, in band 4 (0.76-0.90 mm wavelength, left) and band 7 (2.08-2.35 mm wavelength, right) at pixel sizes of 30 x 30 m. In band 4, much of Etna is snow-covered (white), while the active lava flow is the darkest land feature because of its very low reflectance in this part of the spectrum (very-near infrared). Thermal radiance is confined to a narrow channel near the source and is not evident at this scale. In band 7, the active flow is radiant through most of its length. Bright lines are caused by sensor overload. Courtesy of D. Rothery.

Lava field characteristics. The following is an excerpt from a preliminary report by Wyn Hughes and Mark Bulmer, describing the eruption as of 18 May.

Lava leaving the eruptive vent advanced through a tube system that extended downslope to the foot of the western backwall of the Valle del Bove at 1,850 m asl. Several skylights were spaced at intervals along it. At the break in slope, numerous active ephemeral vents issued new lava-flow units onto the surface of the flow field (figure 48). These did not travel far from their source. Surface activity was otherwise absent within the Valle del Bove; lava was being efficiently transported through tubes toward the flow front. One tube system (with skylights and fume) could be traced through the center of the flow field in the Valle del Bove, toward Val Calanna. At the distal end of the Valle del Bove, several pressure ridges were visible, oriented perpendicular to the underlying ground slope.

Most of the surface activity was occurring in Val Calanna, where intense ephemeral vent activity was issuing new lava-flow units onto the flow-field surface. Lava was being supplied to this area through a series of tubes that descended from the Valle del Bove. Most of the activity in Val Calanna appeared to be supplied by a major tube system that could be traced (by skylights and fume) descending the backwall along its S margin (Salto della Giumenta). A smaller tube system probably supplied some ephemeral vents on the N margin of Val Calanna (S foot of Mte. Calanna).

In Val Calanna, effusive activity was mainly concentrated along the S margin of the flow field, where lava had ponded along the S wall of Val Calanna, and behind the man-made earthen barrier. From there, ephemeral vents in the crust fed numerous new lava-flow units, supplying three regions. Where lava moved directly NE, these were progressively widening the flow field at 1,050 m altitude. Flows that initially moved NE, but then changed to a more easterly direction, were supplying units that flowed around the N margin of the buried man-made barrier. Near the barrier, although active aa-textured flow fronts and channel-fed flow units could be traced on the surface of the flow field, most of the activity that contributed to its widening was supplied from tubes in the previous days' flow units. Ephemeral vents at 1,000 m elevation on the N margin of the buried man-made barrier supplied new flow units that were widening the field to the NE. However, these flow units were abutting the distal levee of the 1852-53 flow field, which was largely hindering the widening. On 18 May, some of these slow-moving tube-fed lavas managed to flow out of Val Calanna, and began the steep descent towards Zafferana. This activity was occurring on the NE side of the flow field. Three ephemeral vents had opened just below the S margin of the man-made barrier. A short distance downslope, flows from these vents combined to feed a front that advanced quite rapidly down the SW side of the flow field on the night of 17 May. By the next morning, and after destroying an abandoned dwelling during the night, the rate of advance had decreased, with the front at ~ 870 m asl. All of these active regions were being channel/tube-fed by lava from along the S wall of Val Calanna, which in turn was being supplied by tubes that descended from the Valle del Bove.

Flow-field morphology. Although the flow field was widening somewhat towards the NE end of Val Calanna, the activity was dominated by ephemeral vents extruding new flow units onto the surface of the original field. This was mainly occurring at ~ 1,800 and 1,050 m altitude, where the backwalls of the Valle del Bove and Val Calanna give way to their respective floors (figures 48 and 50). The surface activity was rapidly burying aa channel-fed flow units from early in the eruption. They could only be seen among the flows that had gone around the N margin of Mte. Calanna, and as isolated inliers on the floor of Val Calanna.

Figure 50. Profile of the pre-eruption terrain in the 1991-92 lava field at Etna. Sites of ephemeral vent activity and zones of lava tubes and channel-fed units are shown diagrammatically. Courtesy of J.W. Hughes and M. Bulmer.

New flow units from ephemeral vents generally emerged with pahoehoe surface textures, in contrast to the early activity whose products had entirely aa textures. The flow-field surface on the floor of Val Calanna, as already occurred in the Valle del Bove, was slowly becoming dominated by pahoehoe textures. Small-scale pahoehoe textures, similar to those described by Pinkerton and Sparks (1978) for the sub-terminal 1975 flow field, prevailed around the ephemeral vents in Val Calanna. However, among the more active vents, pahoehoe slab textures that characterized the near-vent surfaces of new channel-fed flow units progressively changed to aa with increasing distance from the vent area.

Comparison with historical flow fields on Etna. The current ephemeral vent activity within the 1991-92 flow field is consistent with the pattern of historical eruptions that lasted > 100 days (Hughes, 1992). By then, the early channel-fed aa activity that characterized the lengthening and widening phases in the flow field's growth had given way to a tumulus-building phase at the vent area — for example, 1865 (Fouque, 1865); or at a break in slope below the vent area — for example, 1950-51 (Cumin 1954) and 1983 (Frazzetta and Romano, 1984). Important in the emplacement of the 1983 flow field was the evolution of the main supply channel near the vent into a lava tube. By the eruption's 60th day, the tube formed a continuous link between the vent and the lava mound that had accumulated around the break in slope at 2,000 m altitude. The hydrostatic pressures generated within the lava tube were then sufficient to lift and fracture the roof of the lava mound, allowing the escape of lava through ephemeral vent activity. This sequence of events signified the early stages of tumulus development. The present activity occurring at 1,800 m altitude within the Valle del Bove is similar.

The second area of ephemeral vent activity away from the vent area and initial break in slope appears, however, to be unique to the 1991-92 flow field; a similar phenomenon has not been documented for Etna flow fields of the last 250 years. For most, the concave profile of the volcano's flanks (figure 51) meant that once the lava had descended from the steep upper slopes it only encountered progressively gentler gradients. However, the terrain over which the 1991-92 lavas have flowed is much more irregular, with a terraced appearance. The steep terrain around the vent in the upper Valle del Bove is duplicated downslope in the upper reaches of Val Calanna. The morphologic positions of the ephemeral vent activity within the Valle del Bove and Val Calanna are similar (figure 50); both occur at the foot of a steep slope down which lava is transported through tubes. It must be concluded that conditions favoring tumulus construction have also been duplicated within Val Calanna.

Lava production continued from the fissure that opened in the W wall of the Valle del Bove on 15 December. Gas emission from 4 vents in the upper part of the fissure (2,215-2,235 m altitude; figure 52) fluctuated daily, probably with changes in weather conditions. However, gas emission has diminished since the eruption's initial months.

Figure 52. Sketch map of the fissure system and the upper part of the lava field at Etna, June 1992. Contour interval, 50 m. Courtesy of Romolo Romano.

No variation was evident in the movement of lava visible through a skylight high in the main channel, at 2,205 m altitude. Lava was also seen flowing through a skylight in lava tubes that formed in June along the channel into which lava was artificially diverted on 27 May (~ 1,980 m elevation) (17:05). From there, lava advanced through a complex series of tubes past the field that had formed in recent months. Lava again reached the surface around 1,800 m altitude from a changing number (generally 3-4) of ephemeral vents at varying locations representing tube bases. Lava flows extruded from these vents have generally been modest, have remained in the center of the lava field, and have not advanced beyond 1,600 m asl. As of the morning of 9 July, only one flow was active within the Valle del Bove, near the center at around 1,670 m altitude, with a fairly well-fed front. The volume of lava produced during ~7 months of eruption is estimated to be around 165 x 106 m3.

Seismic activity during the period was characterized by low energy release. Significant increases were observed 8-9 July, when events of 2-4 Hz were recorded. The most significant perturbations were detected on 8 July at 1554, for 180 seconds, and at 1601 for 130 seconds. Tremor was almost nonexistent, obscured by seismic noise that characterizes periods of low activity at the volcano.

More or less voluminous gas emissions occurred from two vents at the bottom (~100 m from the rim) of the two central craters (Bocca Nuova and La Voragine). Incandescence caused by superheated gases (>1,000°C) from the vent in La Voragine was sometimes visible. Gas also emerged from a vent that has opened in Southeast Crater. Northeast Crater appeared to have been completely obstructed by internal collapse. COSPEC measurements of SO2 flux from the summit crater showed relatively high values of ~ 8,000 t/d.

Continued lava production from SE-flank fissure; lava diversion summarized

The following, from R. Romano, describes activity from early July through early August.

Early July-early August activity. The eruption ... was continuing after ~ 8 months. Gas emission from the upper part of the fissure has greatly diminished lately, although abundant white vapor was often observed, probably because of weather conditions. Fieldwork on 5 August revealed no notable changes in effusive activity from previous months. The lava flow was visible through a skylight at the beginning of the main lava channel (at 2,205 m asl) and through two smaller skylights at 2,100 m altitude. From there to ~ 1,800 m, lava flowed through a complex system of tubes, resurfacing from numerous ephemeral vents that varied in number (generally about 10) and location (mainly in the center of the lava field). From these ephemeral vents (all between 1,800 and 1,700 m elevation) very modest lava flows emerged. These advanced a few hundred meters at most, never moved past 1,600 m altitude, and remained within the pre-existing lava field. The total volume of lava produced by 234 days of activity was estimated at 170 x 106 m3.

No significant changes were observed at the central craters, where gas emission continued. The more active vent in early August was at the W crater (Bocca Nuova). Northeast Crater has remained obstructed for a few months, with only weak fumarolic activity on the inner walls. Internal collapses continued to occur. Gas emission from Southeast Crater was unchanged.

Seismic activity was low, with only 22 recorded events from early July through early August. The majority of the seismicity was characterized by swarm sequences in the summit area. The most significant, on 11 August, consisted of four shocks with a maximum magnitude of 2.5. Harmonic tremor was of very low energy and showed no variation over time.

The following is from a report by L. Villari.

Civil Protection problems and lava diversion. An earthen barrier was erected at the E end of Val Calanna by the beginning of January 1992, to prevent or delay the advance of lava into a narrow valley leading directly to the nearby (~ 2 km downslope) village of Zafferana Etnea (17:02). Lava expanded into the large Val Calanna basin in February and March, and began to accumulate against the inner wall of the barrier on 14 March. By the end of the month, lava almost completely filled the Val Calanna basin and rose slowly up the barrier's inner wall. Several lobes successively reached the barrier, and the lava field progressively grew and thickened, reaching the barrier rim by 7 April. Lava first overflowed the barrier, along its N sector, during the evening of 8 April, quickly followed by other lobes along the S and central part of the barrier's rim. Lava covered ~ 1 km during the first few hours, merging downslope into a single stream that advanced quickly toward the village. The flow's confinement in a narrow valley favored more rapid progress downslope. Three minor earthen barriers were rapidly constructed along the valley (10-11 April, 830 m asl, 110 m long, 12 m high; 11-12 April, 810 m asl, 90 m long, 6 m high; 13-14 April, 770 m asl, 160 m long, 12 m high) to slow the advancing flow. The barriers were built, like the major one at the E end of Val Calanna, by digging the valley bottom in front of the advancing flow and accumulating the loose material on a small natural scarp. Because the valley is narrow, the confined basins were only able to contain small volumes of lava, and the flow's advance was only briefly delayed (for hours to a day). The front reached <1 km from Zafferana (at Piano dell'Acqua) on 16 April, ~1.5 km from the major barrier and 8 km from the eruptive fissure (figure 53).

At that time, morphologic conditions prevented any other local intervention to slow the lava advance. The creation of any possible artificial obstacle to the advancing front would divert the flow toward inhabited areas not necessarily threatened by the natural flow path. Diversion efforts were therefore concentrated far upslope, near the eruptive vent.

Attention was primarily on a skylight in the main lava tube at ~ 2,000 m altitude on the W wall of the Valle del Bove, a few hundred meters from the active vent. The diversion's early focus was blockage of the main tube carrying lava to the active front, by sliding solid rocks and concrete blocks into the flowing lava. Access problems required transport of solid materials to the site by helicopter, to be directly unloaded into the lava stream, or accumulated around the skylight's rim for later use. Lava tube blockage was also assisted by blasting large volumes of solid lava and welded scoriae forming the flow levees. This was partially successful and contributed to slowing the advance of the active front by several days.

Despite these efforts, on 5 May, a major new flow emerged from Val Calanna atop the 10 April flow, reaching Piano dell'Acqua on 11 May, 120 m beyond the 16 April flow and ~ 500 m from the outskirts of Zafferana. On 22 May, a further attempt to divert lava from the main natural tube to an artificially excavated channel high in the Valle del Bove produced a vigorous lobe that traveled 1 km in a few hours. Only 1/3 of the lava was spilled into the artificial channel, and the new flow roofed over within two days, with a significant loss of supply from the main natural flow.

A four-phase intervention plan was then defined (figure 54): a) digging an artificial channel to drain the main natural tube; b) cutting the lateral tube wall to a minimum thickness (2-3 m) that could be blasted through with a single charge; c) blasting the lateral wall; d) blocking the natural tube to divert all of the lava into the artificial channel.

Figure 54. Sketch of the lava diversion carried out at Etna, 27 May 1992. Courtesy of L. Villari.

Phases a and b were accomplished in about a week. A 7-ton charge, set off in a single explosion on 27 May at 1636, opened a large breach in the natural tube and caused spillage of ~ 80% of the flowing lava. The natural tube was progressively blocked by sliding solid materials into it during the next two days, and the flow was totally diverted into the artificial channel by 29 May. The artificially channeled flow went down the W slope of the Valle del Bove and remained confined inside the valley. The diversion effort stopped the most advanced front that had been moving toward Zafferana, by removing its source of supply.

The artificially channeled lava flow had extended to 1,550 m asl in the S part of the Valle del Bove (at Piano del Trifoglietto) by 30 May. Lava output from the ephemeral vents in Val Calanna quickly decreased, and molten lava was not evident within a few days.

The effusion rate from the eruptive fissure decreased sharply 31 May-1 June, causing the active flow front to be confined within the Valle del Bove, as activity resumed in the central craters. Several hours of continuous ash emission occurred from the W crater (Bocca Nuova) on 31 May, and an incandescent blowhole formed in the E crater (La Voragine) following gas blasts on 1 June. Noisy gas emission continued from La Voragine in succeeding days.

During June, lava flowing in the artificial channel expanded within the Valle del Bove to ~ 1,650 m elevation, overlapping the lava field that had formed since January. The effusion rate was reduced ~ 50% by the end of June, and the upper part of the artificial channel became a tube. The longest flow did not extend more than 1.5 km from the diversion point at 2,000 m altitude. At the end of June, the newly generated lava field, overlapping the old one, covered ~ 0.8 km2.

Northeast Crater. Repeated inner-wall collapses have been observed in Northeast Crater since February. They became quasi-continuous from 26 February through mid-March, associated with explosive activity that ejected blocks and caused a little fine reddish ashfall. From the end of March until 23 May, the collapses were limited to episodes lasting only several hours each, associated with only minor fine ashfall. The crater bottom dropped ~70 m, leaving a pit ~100 m across in place of the previous funnel-shaped depression.

Lava flow measurements. Lava-channel dimensions, flow velocity, and related rheological parameters were observed at a skylight along the lava tube at 2,000 m altitude, and at ephemeral vents in the Val Calanna area, 7 km downstream at 1,000 m elevation. Flow velocities at the exit of the lava tube (~ 4-5 m wide and 5 m deep) in May and the beginning of June were 0.5-1 m/s; flow rates and viscosities were 15-25 m3/s and 100-300 Pas. At the ephemeral vents and the single-channeled flows (1-4 m wide and 1-2.5 m deep), March-May flow velocities were 0.1-0.3 m/s. The calculated flow rate ranged from 0.1 to 4 m3/s, with a corresponding viscosity of 150-1,300 Pas. (See the report by Murray, below, for velocities and flow rates from late June through mid-July).

Direct measurements in June along the main channel (10-40 cm below the lava surface) at 2,000 m altitude, using an immersion thermocouple (Pt-PtRh) yielded temperatures of 1,053-1,068°C. Values were similar (1,030-1,068°C) at several ephemeral vents (10-60 cm inside the lava flow) in the Val Calanna area from March until the end of May.

Petrography and chemistry. Analysis of lava sampled near the vent and at the flow fronts showed no significant variations in chemical or petrologic composition (17:02). All are porphyritic hawaiites (Mg## 52-54), with phenocrysts of plagioclase (15-25 volume %), clinopyroxene (7-10%), olivine (2-3%) and minor (~ 1%) Ti-magnetite.

Seismicity. Low-level seismic activity characterized February-June, despite the continuing eruption. The daily rate was quite low, with only 24 fault-derived earthquakes of M >1 recorded during the period, a rather low value for Etna. No variations were evident in the daily rate or the cumulative strain release (figure 55). Most of the recorded shocks were centered on the SE flank. Maximum local magnitude was 2.8. There were no significant changes in the pattern of volcanic tremor amplitude. Two short episodes of increasing amplitude, on 31 May and 1 June, had maximum overall amplitudes slightly lower than during the December 1991 eruptive phase.

From 26 February until May, seismic stations on the upper flanks recorded many shocks characterized by an emergent onset and low frequency content. At least three waveform types were recognized. All of the shocks were located near the summit craters at <1 km depth. At the same time, morphologic changes were noted within Northeast Crater, associated with the emission of non-juvenile tephra. Most of these shocks were believed to be linked to rockfalls within Northeast Crater. Some explosion shocks were recorded during the same period. These phenomena were most common in February and March, then gradually decreased, disappearing entirely by 23 May.

Ground deformation. Continuous monitoring of ground tilt in a shallow borehole network showed only minor variations since the eruption began in December 1991. No sign of the expected deflation of the volcano was noted, despite the large volume of magma that has been erupted.

EDM networks on the S, SW, and NE flanks, previously surveyed in 1991, several months before the eruption began, were re-measured in late spring and early summer. Contraction was observed, mostly on the SW and NE flanks, while the S flank did not show any appreciable change in line length. The overall deformation pattern of the volcano appears consistent with shallow magma injection into the eruptive fissure, trending roughly NNW-SSE (figure 56). GPS surveys in April-May 1992 detected significant contraction of lines, mostly on the W flank, compared to previous surveys in June-July 1991 (figure 57).

Lava flows. The rate of lava production from the vent in the W wall of the Valle del Bove was much lower than in April. Active flows were visited on 28 June, and 7, 10, 12, and 13 July. Central flow speeds of 2-10 m/minute (depending on slope), widths of 1.5-6 m, and a rate estimated at around 0.3-0.4 m3/s were noted at a single flow on 28 June. A flow about twice as big was seen to the E, suggesting a total discharge of the order of 1 m3/s. Flow fronts were only advancing to ~ 1.2 km from the vent on 28 June, but discharge seemed slightly increased during July visits to the fronts, which were about 2.2 km from the vent on 7 July, and 2.6 km by 13 July.

Summit activity. Continued collapse was occurring around the edge of Northeast Crater, with rockfalls every few minutes or so. Particularly big collapses were seen on 8 July between 1556 and 1610. Southeast Crater had strong high-temperature fumaroles, but no Strombolian activity.

The floors of the two central craters both had single vents that continuously discharged hot gas without any explosions. The vent in La Voragine was ~3 x 10 m, glowed bright red in daylight, and beginning 10 June emitted gas in voluminous puffs from which radiant heat could be felt. There were no signs of fresh bombs or scoriae around the vent. The depth of Bocca Nuova was estimated at ~160 ± 20 m.

Vertical movement. A 25-km levelling traverse, and heights derived from trigonometric levelling during trilateration, yielded details of vertical displacement of 241 stations across the summit and upper flanks since September 1991. Subsidence occurred along a narrow strip extending SSE from the summit, with maximum movements reaching just over 1 m (at two stations between Cisternazza and Belvedere). This central strip is flanked by a swelling to the W of 3-7 cm, and a much larger swelling to the E that reaches 37 cm (at Serra Giannicola Piccola). Southeast Crater has dropped 87 cm and Northeast Crater 48 cm, and the NE rift has risen another 3.4 cm (near Monte Pizzillo). These movements are similar to displacements seen over eruptive dikes in 1989, 1986, 1985, and 1983, but the swelling to the E is higher and broader than any previously recorded.

Horizontal movement. The summit trilateration network shows E-W extensions of 1-1.5 m since September 1991 across the graben and fissures leading S to the eruption site. It is clear that the main feeder dike passes between the Torre del Filosofo and Belvedere, and probably crosses into the Valle del Bove just E of Cisternazza (figure 53). Movements of this magnitude are not unusual during Etna's flank eruptions, and are similar to those recorded during the four eruptions mentioned above.

After network adjustment, some individual station vectors showed unexpected movements. Many of the stations E of the summit also show large eastward displacements, with two (near the Serra Giannicola Piccola) showing 1.3 m of eastward movement, and much of the Valle del Leone having moved 0.5 m ENE. The region at the top of the valley's E wall is cut by new N-S fissures, and SE of Southeast Crater is a region of complex fissuring N of a new cinder cone.

Dry-tilt data. Results from the 30 dry-tilt stations confirm that this eruption is a major one among recent eruptions. In addition to the expected large tilts near the eruptive fissures (192 µrad near Cisternazza), unusually large post-September 1991 tilts of 115 and 92 µrad occurred ~ 4 and 5 km SW of the summit (at Monte Palestra and Monte Vituddi). Unexpectedly large tilts were also recorded ~ 7 km NW and 4.5 km WNW of the summit (at Monte Maletto and Monte Nunziata), and both the Punta Lucia and Pizzi Deneri stations have abruptly increased their tilt to the E, as after the 1981 eruption.

The observed dry tilts are exceptional and suggest that something fairly fundamental has occurred. Only the 1981 eruption had tilts of this size at distant stations. That eruption marked a major turning point in Etna's deformation. After 1981, five stations that had previously been stable, even during flank eruptions, tilted during the next few years by amounts that eventually totalled as much as 1,000 µrad.

The eruption ... appears to have become more vigorous in early September. Until 2 September, activity was similar to previous months, with lava flowing through a complex system of tubes and resurfacing from ephemeral vents of varying number (recently about 4) and location. Lava flows from the ephemeral vents were generally of modest size, advancing only a few hundred meters and remaining within the pre-existing 1991-92 lava field. Gas emission from the upper part of the fissure has remained similar to past months, with modest variations linked to weather conditions.

The roof of the main lava tube broke open late in the evening of 2 September, near a former skylight at ~2,150 m altitude and above the site of artificial lava diversion in May (at ~2,000 m elevation; 17:07). The flow from the broken tube first moved ENE above the lava field, then along its S side over terrain not previously covered by lava from this eruption. Lava advanced ~ 1 km to 1,700 m altitude by the next morning, then slowed to a stop during the succeeding days, reaching ~1,650 m elevation. The main lava tube, with two fuming skylights, remained active on 4 September below 2,000 m altitude. The only large ephemeral vent was at around 1,700 m asl, in the central part of the lava field. Another overflow occurred during the morning of 9 September at around 1,980 m altitude, below the lava diversion area. The flow advanced across the central part of the lava field, quickly reaching 1,650 m elevation. The total volume of lava produced by 272 days of activity was estimated at 190 x 106 m3.

Activity from the central crater's two vents remained similar to previous months, with continuing gas emission, strongest from the W vent (Bocca Nuova). Gas, sometimes voluminous, also continued to emerge from two small vents in Southeast Crater. Northeast Crater remained obstructed by debris. Landsliding persisted from the walls, particularly in the N and S parts of the crater. Very weak fumarolic activity occurred from Northeast Crater's inner walls. SO2 flux from the summit craters, measured by COSPEC, remained at relatively low levels (~ 2,000 t/d) during August, but measurements in early September yielded higher values, ~ 5,000 t/d.

From 15 August through 11 September, 176 seismic events were recorded, characterized by low energy release. The most significant activity was a sequence of 43 summit-area events 6-7 September, with a maximum magnitude of 3.2. Harmonic tremor has been nearly absent.

The SE-flank fissure eruption ... continued relatively unchanged in September and early October 1992. Gas emission from the upper part of the fissure was similar to previous months, varying with weather conditions. Lava continued to flow through a complex tube system, emerging from ephemeral vents at frequently changing locations. The resulting lava flows were generally modest-sized, advancing only a few hundred meters over the pre-existing lava field. This pattern of activity changed only when a substantial increase in the amount of lava moving through the main tube caused an overflow through a skylight. On 3 October at about 1830, lava began to emerge from a skylight at 2,150 m altitude, preceded by vigorous emission of white vapor. The overflow remained active on 8 October, and lava had advanced about 1 km. A similar episode occurred from the same location in early September. During 8 October fieldwork, numerous ephemeral vents were also active. Three were in the area of May's artificial lava diversion around 2,000 m altitude, three at ~ 1,800 m elevation (around Serra Pirciata), and 3-4 others near 1,700 m asl. Flows from the ephemeral vents remained modest in size, did not advance beyond 1,650 m altitude, and stayed within the Valle del Bove. Total lava volume from 300 days of activity was estimated at around 210 x 106 m3.

Gas continued to emerge from two small vents on the floor of the central craters, at ~ 100 m depth. Gas emission generally occurred under pressure from the W crater (Bocca Nuova). A small vent on the S edge of Southeast Crater continued to emit gas. Northeast Crater remained obstructed by debris, with landslides still occurring in its N and S parts.

SO2 emission, measured by COSPEC, continued to increase. During the first 10 days of October, values exceeded 10,000 t/d, twice Etna's average SO2 flux.

Seismicity remained at low energy between 11 September and 13 October. About 230 microearthquakes were recorded, centered mainly in the summit area. The largest (M 3.0) occurred on 16 September at 0650 and was felt in nearby towns. On 27 September, a brief sequence of 7 events occurred on the W flank. The strongest shock (M 3.7), at 1255, was felt to ~ 70 km away (in the Siracusa area). Tremor has been nearly absent.

The eruption ... appears to have become slightly stronger in recent weeks. A small increase in the effusion rate was apparent during the report period (13 October-12 November) at the main vent (2,210 m asl) after lava production had remained relatively constant for the previous several months. Lava initially moved through a single tube, with a surface trace marked by four skylights from 2,210 to 2,150 m altitude. The lava resurfaced (beginning at ~1,780 m elevation) within the lava field formed in past months. Three large ephemeral vents fed wide, thick flows, some of which advanced more than a kilometer within the lava field. On 11 November, the front of one flow was at 1,600 m altitude, in the center of the lava field. Small flows also emerged from tens of minor ephemeral vents, with locations that changed daily. Characteristic cumulo-domes formed in areas with high concentrations of ephemeral vents. The total volume of lava produced by 334 days of eruption was estimated at ~ 240x106 m3.

Gas emission from the upper part of the eruptive fissure was also a little stronger than in previous months. Fluctuations in the apparent gas emission rate remained linked to weather conditions. Vigorous degassing continued from Southeast Crater and from the central crater's two vents. Rare, modest ash ejections occurred from the W vent of the central crater (Bocca Nuova). Weak fumarolic activity continued from the walls of Northeast Crater, still obstructed by debris. SO2 flux, measured by COSPEC, remained high, ranging from 5,000 to 10,000 t/d, typically around 8,000 t/d.

Seismicity remained at low levels 13 October-12 November. About 100 events were recorded, mainly in the summit area, with magnitudes of 1.1-3.4. A large proportion of these occurred during the first week in November. The 34 events detected 3-4 November included a swarm of 15 summit-area shocks between 0500 and 0537 on the 3rd; the strongest, at 0500, had M 3.0. Of the four summit earthquakes recorded between 1220 and 1249 on 9 November, three had magnitudes exceeding 2.5, including the strongest of the report period, M 3.4, at 1249. Harmonic tremor has been nearly absent.

Lava production became more vigorous during the first half of November, but seemed to be decreasing in early December, a year after the SE-flank eruption began. After emerging from the vent, lava initially flows through a single tube, with a surface trace (from ~2,210 to 1,980 m altitude) marked by at least five skylights. Lava then continues into a complex of tubes, emerging from numerous ephemeral vents (which varied daily in number and location) onto the extensive lava field that has developed in past months. On 9 December, the ephemeral vents formed a linear zone between 1,700 and 1,600 m altitude, feeding small flows that did not advance below 1,580 m. The larger flows advanced NNE on the N side of the lava field, while other flows moved mainly toward the E. After 361 days of activity, the eruption's total lava output was estimated at 255 x 106 m3.

Gas emission from the upper part of the eruptive fissure was less vigorous than in early November, with fluctuations linked to weather conditions. Small ash ejections from the central crater's W vent were observed, particularly during the first few days of December. Degassing from the summit craters was otherwise unchanged, and Northeast Crater remained obstructed. SO2 flux, measured by COSPEC, remained at high levels, ranging from 6,000 to 10,000 t/d and averaging ~8,000 t/d.

Between 13 November and 8 December, 140 microshocks were recorded at Etna, mainly in the summit area. Energy levels remained low and magnitudes did not exceed 2.9. Several small seismic swarms occurred. The most vigorous, on 28 November between 0624 and 0858, included 10 events of M 1.7-2.9 centered in the summit area. The number of shocks increased briefly 28-30 November, when 62 were recorded. No harmonic tremor was detected.

The eruption ... continued without major changes through early January 1993. Lava frequently flowed NE and NNE, and by early January there had been a notable expansion of the upper part of the lava field toward the NE. A new lava overflow was observed at the end of 1992 and the beginning of 1993 along the main lava tube above 2,000 m elevation. The overflow fed three distinct lobes that advanced a few hundred meters, to ~ 1,900 m asl. Lava continued to flow through the main tube, and was visible in early January through three skylights between 2,210 and ~ 1,900 m elevation. From there, the main tube divided into a complex tube system, from which lava emerged onto the surface at ~1,700 m altitude through small ephemeral vents. On 8 January, about six ephemeral vents were visible, feeding small flows that advanced NNE and E over the lava field formed in previous months. The NNE flow reached 1,650 m elevation, the E flow 1,600 m. The volume of lava produced by 394 days of activity was estimated at ~ 270 x 106 m3.

Gas emission from the upper part of the eruptive fissure was not very intense. Apparent fluctuations were linked to weather conditions. Summit-crater degassing was similar to the previous month. Northeast Crater remained obstructed, with only weak fumarolic activity on the inner walls. SO2 flux, measured by COSPEC, was still relatively high at ~ 7,000 t/d.

Of the 83 seismic events recorded 9 December-11 January, 51 occurred in four swarms, all located in the summit-crater area, with magnitudes of 1.5-3.3. The most significant swarm, on 2 January, had 17 events, with the strongest (M 3.3) at 1342. This swarm was accompanied by a slight increase in the amplitude of tremor, otherwise nearly absent during the period.

The eruption ... is now Etna's longest flank eruption of the 20th century, surpassing the 372 days of E-flank activity in 1950-51. However, dominantly effusive eruptions from the summit area's Northeast Crater have persisted for many years (May 1957-February 1964; January 1966-April 1971; and September 1975-January 1977) and intermittent explosive activity from the central crater has continued since 1979.

The most active flows advanced NE and NNE, extending the upper part of the 1991-93 lava field toward the NE. On the morning of 4 February, lava flowing in the main tube was visible through two skylights, and emerged from small ephemeral vents on the N and S sides of the lava field. The approximately five northern ephemeral vents, between ~1,900 and 1,600 m elevation, were the most impressive, and fed the strongest flows, to the NNE. The small S vents, two of which were very close to the S wall of the Valle del Bove at 1,550 m asl, were the sources of very modest flows that moved E. Flows from both sets of vents advanced over the pre-existing lava field, and did not extend beyond elevations of 1,600 m (N vents) and 1,550 m (S vents). The volume of lava produced by 429 days of activity was estimated at 280 x 106 m3.

Gas emission from the upper part of the eruptive fissure has declined notably, and as of mid-February only the former explosive vent at the fissure's lower end (2,215 m elevation) remained active. Degassing from the summit craters was similar to previous months. Modest ash emissions, caused by internal rockfalls, occurred rarely from the central crater's W vent. During the early morning of 3 February, phreatic explosions from Northeast Crater ejected old lava fragments to tens of meters W of the rim. A modest ashfall occurred on the E side of the crater, and ash was still visible on the snow during the following days. Northeast Crater was obstructed again after this activity, and the next day only vigorous fumarolic activity was noted on the crater floor. SO2 flux, measured by COSPEC, declined from ~ 7,000 t/d in December to 5,000-6,000 t/d in January, about average at Etna.

Seismicity remained at low energy levels during the report period (12 January-15 February). All of the 125 seismic events (M 0.7-3.4) recorded during the period were centered in the summit-crater area. The seismicity included only one swarm (23 events, maximum M 3.4) on 3 February between 0527 and 0623. All were low-frequency events (1-5 Hz) and occurred as wave-trains that resembled spasmodic tremor. With that exception, volcanic tremor was absent.

The following information, based on the report of the IIV, covers the period December 1992 through February 1993.

The eruption ... continued as lava gently flowed from the vent on the W wall of Valle del Bove, significantly expanding the flow field formed after the flow diversion of May 1992 (17:05). Lava moved to Piano del Trifoglietto through a forked lava tube, emptying through several ephemeral vents located mainly on the N and S sides of the flow field (figure 58). In the first half of December, lava escaped mainly through the S vents. Many small flows gradually covered Poggio Canfareddi Hill, previously isolated by flows moving E toward Mt. Zoccolaro. In the second half of December, activity shifted to the N vents, expanding the flow field over a flat area that had not been covered by lava from the current eruption. Using data from a GPS survey done in January, the total volume of lava erupted through 1992 was estimated to be 198 ± 40 x 106 m3. The lava covers an estimated 6.7 x 106 m2 and the mean rate of lava production is 6m3/s.

Figure 58. Topographic sketch map of the active portion of the 1991-93 lava flow field; 1. Flow field formed from 27 May 1992 through February 1993; 2. Flow field before 27 May 1992; 3. Limit of active lava flows by November 1991; 4. Directions of the main active flows December 1992-February 1993; 5. Lava tubes. Courtesy of IIV.

By January 1993, lava flows from the S vents advanced to the Poggio Canfareddi area and a complex network of minor flows reached the foot of Mt. Zoccolaro at 1,530-m elevation. Lava continued to flow from the N vents, expanding the field 400 m to the N. On 27-29 January, a fast-moving lobe of lava flowed to the NE, reaching 1,500 m elevation, 4 km distant from the eruptive fissure.

Effusive activity declined in February, ceased at the S vents by 8 February. Flow from the N vents was less than in previous months and shifted to vents on the northern-most side of the flow field. The new flows did not expand the flow field.

The seismic network recorded five swarms of long-period events. Fourteen events with M >1 occurred on 1 December, 14 events on 23 December, five events on 25 December, 51 events on 30 December-2 January, and 26 events on 3 February. No event exceeded M 3. The swarms were located in a small focal volume between the summit craters and Pizzi Deneri (~2 km NE) at depths asl. Volcano-tectonic seismicity during the period remained low (only three events) comparable to that observed throughout 1992.

The 9-station bore-hole tiltmeter network recorded no significant deformation except for a sharp event 18-19 December. Steady degassing from the Etna summit craters was observed and a weak ash emission occurred on the morning of 3 December from the Bocca Nuovo vent. Minor landslides repeatedly affected the E inner wall of the NE crater until January. The crater floor had sunk by early morning on 3 February.

The following report from geologists at the IIV and the Univ di Catania with seismic information from G. Luongo, updates and complements the official IIV report above.

A lava flow, at least a few hundred meters wide, has formed on the NE side of the lava field that has been building since 27 May 1992 (figure 58). The flow, in the vicinity of Monti Centenari (2 km NE of the active fissure), is completely independent from the old field and is moving E in the middle of Valle del Bove. The lava of this flow is visible from between 2,205 and 1,700 m through a series of skylights on the main tube. Lava is surfacing through 4-5 ephemeral vents at ~1,500 m elevation; the vents active in the past on the N, S, and central parts of the old lava field have closed. On the morning of 12 March, the most advanced flows had reached 1,425 m elevation and were moving over lava of previous eruptions. By 1300 on 14 March, the lava front was at 1,400 m elevation, ~ 5 m wide, 1 m high, and moving at an estimated 1 m/h.

The estimated volume of lava produced after 458 days of activity is 295 x 106 m3. This estimate was calculated using the same method as previous estimates reported in the Bulletin, but is ~ 50% higher than the GPS value reported above. No significant changes in degassing of the summit craters were noticed. Northeast crater is still obstructed, with very active fumaroles along the inside walls. COSPEC measurements of SO2 flux remained normal (5-6 x 103 t/d), except in early March when measured values were 12.5 x 103 t/d.

Between 12 February and 15 March, 169 events of M 1.2-2.9 were recorded, mainly in the summit crater area. The majority of events appeared to be related to active degassing at the surface, with characteristic frequencies of 1-6 Hz. Volcanic tremor was completely absent.

The following, based on the report of the IIV, describes activity in March.

The 1991-93 eruption ended on March 30 after 473 days of continuous lava flows. Lava stopped flowing to the S side of the flow field in mid-February, but continued to flow from several vents at 1,700-1,650 m elevation on the N side of the field, covering previous flows from the current eruption. Shortly after 8 March, an overflight revealed a thin flow that had traveled 0.5 km from an ephemeral vent at 2,020 m elevation. On 11 March a large lava flow moved toward the Valle del Bove over an area not yet covered during this eruption. Flowing 1.5 km from a large ephemeral vent at the end of a tube at 1,550 m elevation, lava spread down some gullies, stopping after 3 days at 1,390 m elevation, 5 km from the eruptive fissure (figure 59).

Figure 59. Topographic sketch map of the active portion of the 1991-93 lava flow field; 1. Flow field formed after 27 May 1992 through March 1993; 2. Flow field before 27 May 1992; 3. Limit of active flows through February 1993 ; 4. Directions of the main active lava flows in March 1993; 5. Lava tube. Courtesy of IIV.

Lava flows declined in the second half of March. On 21 March a vigorous flow emerged from a vent at 1,850 m elevation and traveled several hundred meters NE in a few hours before slowing and finally stopping within two days. The remaining ephemeral vents gradually disappeared and the last small lava flow was observed on the morning of 30 March. In the following days, fieldwork at the eruptive fissure and along the upper lava tube revealed that no molten lava remained in the tube and that the delivery system was cooling.

Steady degassing continued through March at the summit craters. The floor of Northeast crater dropped another several meters. Seismicity remained low, especially from 1-10 March. For the month, 65 long-period events and 8 volcano-tectonic events (maximum M 2.4) were recorded. The only notable swarm occurred on 28 March when 10 events were recorded in two minutes. There were no significant variations in the volcanic tremor amplitude. Four of the 9 bore-hole tiltmeters recorded a sharp deformation event of moderate amplitude at the beginning of March.

The 1991-93 eruption began on 15 December 1991 and lasted 473 days. It was probably the largest eruption at Etna in the last 300 years, covering ~ 7 km2 with >250 million m3 of lava.

The following information compiled by volcanologists at the IIV, Univ di Catania, and OV complements the official IIV report above.

The lava flow that had reached 1,400 m elevation on 14 March (18:02) stopped on 17 March. At about 1700 on 21 March a modest lava flow escaped through a skylight on the main lava tube just below 2,000 m elevation. It was accompanied by intense degassing from the upper part of the eruptive fissure. Through 25 March lava was observed flowing inside the main vent at 2,205 m and small, short-lived flows issued from ephemeral vents in the N part of the lava field at ~ 1,900 m elevation. Poor weather prevented detailed observation of the last days of the eruption.

Degassing (vapor and gas) from the upper part of the eruptive fissure declined. By 20 March it was difficult to observe from a distance. Degassing increased at the summit craters (especially from the central crater) during the final phase of the eruption. Through 9 April, the NE crater, where recent rockfalls had occurred, continued to be obstructed and weak fumarolic activity was present along the walls. COSPEC measurements of SO2 flux revealed a sharp increase during the last half of March (>16 x 103 t/d on 23 March). Measurements in April indicated the flux was returning to a normal level of 6-8 x 103 t/d.

From 16 March to 19 April, 337 seismic events were recorded. They ranged from M 1.0-3.0 and showed characteristic frequencies of 1-6 Hz. All were located in the summit crater region except a M 2.7 at 0649 on 14 April that was located low on the E flank. Volcanic tremor was totally absent.

During the 1991-93 eruption an estimated 300 x 106 m3 of lava flowed from the fissure on the W wall of Valle del Bove at an average rate of 7.3 m3/s.

Information Contacts: The first section is from the official report of theIIV. The second section is fromR. Romano, T. Caltabiano, M. Grasso, and M. Porto, IIV; P. Carveni and C. Monaco, Univ di Catania; G. Luongo, OV.

Steady degassing from the summit craters followed the end of the 1991-93 eruption on 30 March (18:03). Increased gas emissions were noted at the central (Voragine) and SE craters (see figure 59) in April, but no morphological changes were detected. The floor of Northeast Crater sank a few meters in early April and remained obstructed by fallen material.

Seismic activity was low with only two volcano-tectonic events recorded. The highest magnitude event (M 2.7) occurred 14 April on the SE flank of the volcano at ~ 10 km depth. Long-period events were similar to those recorded in March, but fewer in number. There was also a decreasing trend in volcanic tremor spectral amplitude. No major changes were recorded by shallow bore-hole tilt stations on the slopes of the volcano.

Following the recent eruption's end in March, steady degassing was observed in May from all of the summit craters except Northeast Crater, which remained obstructed by debris in the bottom. Seismicity increased from two volcano-tectonic events in April (18:04) to 23 in May (M 1-3.2). Most of the events occurred between 1206 and 2039 on 24 May. At the same time, an earthquake swarm with 21 discrete events occurred below the NNW flank of the volcano at 13-26 km depth. The number of long-period events also increased compared to the last two months, but tremor amplitude and frequency were unchanged. Inflation was detected at one shallow bore-hole tilt station, but no other tilt variations were recorded.

Only steady degassing has been observed at Bocca Nuova, Voragine, and Southeast summit craters following the December 1991-30 March 1993 eruption. Northeast Crater, obstructed by debris that fell from the inner wall, has not shown appreciable degassing.

On 3 August 1993 the Bocca Nuova bottom sank ~30 m during one hour of strong degassing and ash emission that produced an ash column hundreds of meters high; small blocks and a few fresh bombs fell close to the vent. Unusually strong noise was heard and ground vibration was felt at the summit area during this explosive activity. These phenomena also enlarged the unstable crater rim, causing rockfalls for several weeks. Activity did not change significantly through the end of 1993; continuous degassing activity was observed at all craters except Northeast Crater, where reddish ash emissions in early October were probably related to release of overpressurized gas.

A slight renewal of seismicity was observed after the end of the eruption. Fracturing was the probable cause of 83 events (M >1); 14 of them were M 2.5. The cumulative strain-release trend was almost flat throughout the entire period, the only significant episode was a seismic swarm on 24 May 1993 (twenty-one M 1 shocks; Mmax = 3.2). The seismic activity was mainly located on the N and SE sides of the volcano; the N events had hypocentral depths of 12-26 km, whereas the SE events were <10 km. Volcanic tremor amplitude remained low during 1993; a moderate increase was recorded in July. Also, 27 long-period earthquake swarms were recorded in 1993. The best constrained hypocentral locations revealed a source volume below the summit area at a depth of <=3 km.

Tilt recorded at most of Etna's bore-hole stations showed a continuous small deflation of the radial component that started during the 1991-93 eruption. This tilt was confirmed by general contraction measured by the three EDM networks.

The following report is from S. Saunders and W.l McGuire. An EDM network high on the S and E flanks has been reoccupied 13 times between 1981 and 1993. Measurements have revealed >5 m of lateral displacement associated with four rifting events. The network was at least partly re-occupied in April, July, and November 1993. All three surveys came after the cessation of effusive activity in March 1993 (18:03). Compared to the immediately preceding measurements, 1993 data showed that N-S trending lines, broadly parallel to the eruptive fracture and the W rim of the Valle del Bove, lengthened by small amounts (30-60 ppm). Lines trending E-W, perpendicular to the fracture zone, showed no significant length changes between November 1992 and November 1993. These data confirm that the rifting process is contemporaneous with the initial propagation of the feeder dike for the 1991-93 eruption, with little additional dilation-related lateral displacement during the later stages of activity or following the end of lava effusion.

"As during visits in June-July and September-October 1993, Northeast Crater was blocked and inactive, but collapse was continuing around the edge with minor rockfalls every few minutes or so. Southeast Crater was also little changed from 1993, with a quietly degassing vent under the SE rim, but no indication of gas coming out under pressure. There was strong high-temperature fumarolic activity around the crater rim, temperatures being generally highest in the cracks.

"The Chasm (La Voragine) had a single vent in its floor measuring ~ 8 x 10 m, discharging gas continuously under pressure in rhythmic puffs at a rate of ~ 30 puffs/min. On 17 June and 12 July only, distinct explosions could be heard at the rate of 1-8/min. These were the first signs of explosive activity since the end of the 1991-93 eruption, and an indication that the Strombolian degassing that has characterized the summit over the past few hundred years is resuming.

"Bocca Nuova vent was degassing almost totally silently from two vents, one to the SE and one to the W; however, on 27 June when the weather was calm, 13 very faint gas puffs/min could be heard. The SE vent seemed similar to last year, measuring ~ 10 m in diameter, but the W vent had collapsed and enlarged considerably, now measuring perhaps as much as 50 m in diameter. On the early morning of 16 June a reddish tinge to the plume above Bocca Nuova was first noticed. Upon closer inspection on 17 June, the SE vent was seen to be pouring out thick clouds of red dust, apparently a result of internal collapse within the vent, while the W vent continued to emit white fume only. Dust emission intensified in the following days, causing the downwind side (S through W) of the summit to become a striking red color. The activity was continuing in mid-July.

"The levelling traverse showed comparatively small vertical movements since September 1993. The area near Belvedere, and other areas over the dyke intruded during the 1991-93 eruption, had subsided by up to 2 cm, as had the NE rift zone near Monte Pizzillo. During the same period, a small area ~1 km SW of the summit inflated by just over 1 cm. Horizontal movements measured since October 1993 showed generally small or insignificant changes, with nearly all lines recording changes of >1 cm. Only two stations appear to have moved by more than this; a station on the E edge of Southeast Crater had shifted 3 cm E relative to nearby stations, and a station close to the NW edge of the Bocca Nuova had moved 2 cm W. These movements are consistent with expansion of the central magma column as it refills.

"Surface temperatures were measured between 1 and 27 June at four active fumarole areas with a Minolta/Land Cyclops Compac 3 hand-held radiometer (8-14 mm). Temperatures were not corrected for spectral emissivity, so all radiant temperatures are given here as brightness temperatures. On the NE rift zone, nine areas of fumaroles were observed near the N edge of the 1966-67 lavas (between 2,450 and 2,500 m altitude). Temperatures for fumaroles at the two lowest of these areas ranged between 33 and 50°C. Another area of fumaroles observed at the upper rim of the W wall of the Valle del Bove around Belvedere, above the 1991-93 dyke, had temperatures in the 57.5-84.7°C range. Temperatures measured at fumaroles and cracks in the still-cooling 1991-93 lava-flow field in the Valle del Bove were between 85 and 221°C. The locations and temperatures of fumarole areas measured in the vicinity of the summit craters are given in table 5. Temperatures of the vents within the central craters were also measured from the crater rim: 342°C for the Chasm vent, and 159 and 81.5°C, respectively, for the SE and W vents of Bocca Nuova. Active fumaroles were observed, but not measured, along the 1991-93 fissure zone and 14 December 1991 cones and flows between Southeast Crater and Belvedere, along the October 1986 fissure zone, and in the Valle del Bove below Monte Simone."

Table 5. Fumarole temperatures in the vicinity of Etna's summit craters, measured on 18 and 27 June, and 14 October 1994. Courtesy of Andrew Harris, Open University.

The following describes [fieldwork] between 23 September and 14 October 1994.

"There are continuing signs that activity is increasing. At the Chasm (La Voragine), 1-4 very low rumbles/min were heard, but on 14 October six explosions much louder than those heard in June/July (19:07) were heard in 10 minutes. The Bocca Nuova was also producing around one distinct long explosive blast per minute, as opposed to the faint gas puffs heard in the summer. However, no audible explosions were heard when the Chasm was active on 14 October. Northeast and Southeast craters were quiet as in June/July, but temperatures more than 100°C higher were measured at the fumaroles on their outer slopes. Another sign of increasing activity was that during the five days of levelling (25-30 September), 22 earth tremors were detected by the shaking of the instrument. This is > 10 times higher than 1993, and the largest total of tremors noted in this way since September 1991, before the 1991-93 eruption.

"The levelling traverse showed a slight subsidence of the summit since June 1994, the maximum value being just under 3 cm compared to the Piano Provenzana, 6.5 km NNE of the summit. The subsidence is more or less concentric around the summit, with the exception of some stations on the upper E flank and over the 1991-93 dyke, which have subsided nearly a centimetre more than those nearby.

"On 14 October the areas of active fumaroles measured during June were visited. These were measured again using a Minolta/Land 330 hand-held radiometer (8.5-14.5 mm). Temperatures were not corrected for spectral emissivity, so all radiant temperatures are given as brightness temperatures (table 5). At the N, W, and S rim of Northeast Crater, maximum fumarole and rift temperatures were 105-135°C higher than those measured in June. H2S was also smelled in the vicinity of these high-temperature fumaroles. Higher maximum temperatures were also measured from rifts at the N rim of Southeast Crater, these being up to 170°C higher than those measured in June. It is stressed that these rises in temperature may be the result of different fumaroles being measured on the two dates, though in view of the thorough coverage in June this seems unlikely. Elsewhere, fumarole temperatures were similar to those measured in June. Fumarolic activity only was observed on the floor of Northeast Crater, which was measured from the rim at 40.1°C. The bocca on the floor of the Chasm was measured from the crater rim at 339°C. At the Bocca Nuova, a temperature of 173°C was measured for the SE bocca and of 40.7°C for the NW floor; these were measured from the crater rim. At Southeast Crater, fumaroles decreased in temperature and number around the W and E rims, such that fumaroles were few and cool on the S rim."

The following report from the Istituto Internacionale di Vulcanogia (IIV) describes activity from December 1994 to June 1995. Additional information came from Open University geologists, from Henry Gaudru (SVE), and fromaviation notices. Fumarole temperatures measured by Open University geologists in the vicinity of the summit craters increased at Northeast Crater (NEC) between June and October 1994 (table 6). Temperature increases were greatest at the fumarole field on the S rim of the crater, and decreased towards the N rim.

Table 6. Changes in maximum fumarole temperatures measured at Etna's summit craters between June and October 1994. Courtesy of Open University.

Crater Area

Location

June 1994 Maximum Temp (°C)

October 1994 Maximum Temp (°C)

Temperature increase (°C)

NE Crater

Fumaroles at N rim

65

77

12

NE Crater

Rifts at NW rim

141

246

105

NE Crater

Fumaroles at W rim

97

210

113

NE Crater

Fumaroles at S rim

86

221

135

Bocca Nuova

Fumaroles on N flank

76

76

0

Bocca Nuova

Fumaroles and rifts (N rim)

74

74

0

Bocca Nuova

Fumaroles at SW rim

66

72

6

Central Craters

Fumaroles at S rim

83

82

-1

Central Craters

Between S rim and SE crater

81

83

2

SE Crater

Fumaroles and rifts-N rim

312

482

170

SE Crater

Fumaroles and rifts-W rim

208

218

10

After several months of steady degassing from the summit craters, Bocca Nuova produced a short sequence of mild explosive events on 10-12 December 1994, characterized by brownish columns of non-juvenile ash rising

In January 1995 several ash puffs from NEC were observed. They were more frequent between 31 January and 3 February, but continued all month, forming a thin ash layer around the crater rim. The most significant activity from NEC in the following two months was strong steam degassing, sometimes with ash.

An intense episode of ash emission from NEC occurred at 1000 on 9 May. Red-brown ash and accretionary lapilli fell on Milo, a village on the middle slope of the volcano. No block fallout was observed near the crater rim, and steam emission continued unchanged.

On 23 May at 1605 a new NEC explosion ejected lithic blocks; most of them were affected by fumarolic alteration that changed hard lavas and scoriae into very brittle materials with vivid white, yellow, purple, and reddish colors that were very easy to recognize on the discontinuous snow mantle. The area of fallout was ~0.2 km2 and the maximum block volume reached 0.2 m3, however, most of the blocks were only a few centimeters in size. No juvenile material was found among the fall products and the event resembled to a pure phreatic explosion that ejected very altered material picked up from the walls of the December 1994 degassing vent and the NEC crater bottom. On the morning of 26 May an explosion visible (by SVE members) from the N flank at 1,800 m elevation generated a gray ash-and-vapor plume above NEC. When the SVE group reached the summit area, small blocks were visible around NEC and near the lower slope of Bocca Nuova.

On 30 May a weak, ash-bearing plume was observed from an airplane by J.B. Murray. Stronger activity from the vicinity of Bronte was noted on 8 June, when thick ash clouds up to 70 m high were reported late in the morning. On a 12 June summit visit, scattered wall rock (lying

The IIV reported gas explosions and inner-crater wall collapses from Bocca Nuova in June. Gas emission came from two vents on the crater bottom, the northernmost of which produced some small phreatic explosions that threw several centimeter-size lithic-lava blocks up to 50 m NE beyond the crater rim. Some ash emission from NEC was observed during June. Murray reported that as of mid-June guides had stopped taking tourists to the crater edge because of the danger from explosions. The situation reminded Murray of the activity following the 1983 eruption (SEAN 08:04), when a series of sudden, large non-magmatic explosions occurred from the NE crater.

Aviation notices (SIGMETs) were issued for Etna on 21 June when an ash cloud reportedly rose 4,200 m. Another notice on 25 June described an ash cloud ~18 km E from the central crater at an altitude of 2,100-4,200 m. IIV video surveillance showed no eruptive columns during 21-25 June 1995, although on 21 June the camera was out of order and on the afternoon of 23 June foggy conditions obscured the upper slopes. On 22 June light ash from NEC fell on the IIV high-mountain observatory at Pizzi Deneri (2,850 m elevation), NE of the summit craters.

Gas-and-ash explosions followed by sustained Strombolian activity in late July

Eruptive activity from Etna's summit craters continued in July. Sustained Strombolian activity resumed in Bocca Nuova vent on 29 July. Details will be provided next month.

At Bocca Nuova crater, a degassing vent in the N part of the crater floor produced very frequent gas explosions followed by collapses inside the vent, and then by red ash emissions. Ash emissions started on the morning of 25 July, and ash formed a thick carpet inside the crater. Ash plumes rose ~100 m above the crater rim and caused ashfall on the W flank. A small percentage of juvenile material was identified in the ash. Another vent at the SE margin of the crater floor produced only gas explosions with no ash. The interior of Bocca Nuova had vertical walls and a nearly flat zone in the SE part of the floor, ~100 m below the crater rim, that gently sloped NW. The flat zone occupied 40% of the crater floor, the remaining part being covered by collapse debris. The inner part of the collapsed zone had sub-vertical walls and a floor sloping NE.

In the SW corner of the Northeast Crater floor a vent produced strong gas emissions with occasional inner collapses and red ash expulsions. Samples of ash showed an increase in juvenile component compared to May. Northeast Crater was pit-shaped, with sub-vertical inner walls covered by red ash. The floor was ~150 m deep and had a step oriented NE-SW, which separated the flat zone of the NW sector, gently sloping SE, from the SE portion sloping SW. Activity at the funnel-shaped Voragine (Chasm) consisted of continuous, weak gas emission from the central vent with neither explosions nor ash emissions. Southeast Crater produced only a weak degassing activity.

Strombolian activity resumed at Bocca Nuova vent on 30 July and in Northeast Crater on 2 August. Etna's last magmatic activity within its summit craters stopped 3 years and 7 months earlier, two days after the beginning of the 1991-93 flank lava flow eruption (BGVN 16:12). During that 15-month-long eruption and the following 28 months, the four summit craters exhibited continuous steam emission with frequent non-juvenile ash puffs, several collapses, and some strong phreatic explosions from Northeast Crater and Bocca Nuova. No morphological changes were observed in August at either Voragine or Southeast craters, where gas and steam emissions continued as in previous months.

On 30 July the first red spatters were observed inside Bocca Nuova, but bad weather prevented an evaluation of the intensity of this new magmatic activity. Observations the next day revealed that the vent was located in a new pit crater (20-30 m wide and ~50 m deep) on the N part of the crater floor. That part of the crater floor collapsed in June 1994, and probably dropped again in June 1995 when some phreatic explosions occurred (BGVN 20:06). The vent was a few meters across, and magma was sometimes visible during pulsed degassing episodes frequently interrupted by mild Strombolian explosions. The most energetic events were lava jets lasting 15-20 seconds that threw large spatters 120-130 m above the vent to the crater rim. The activity climaxed on 2 and 3 August when lava jets frequently rose up to a few tens of meters above the crater rim. Strombolian activity stopped abruptly on the night of 4 August, leaving a thick tongue of lava on the pit floor. During the Strombolian phase no spatter fell beyond the crater rim; most fell close to the vent inside the inner pit. In the days following 4 August, several ash emissions were observed at Bocca Nuova, which gradually resumed its quiet degassing. No further activity at Bocca Nuova was observed through the end of August.

Until 2 August, no lava emissions had been observed from Northeast Crater since September 1986 (SEAN 11:09); only scoria was ejected on 13 May 1991 (BGVN 16:07). Strombolian explosions during 2-3 August issued from a small vent in the lowest part of the crater, ~150 m below the crater rim. Almost continuous spatter ejections never reached the crater rim. During 3 August the activity gradually changed to puffs of black ash that continued in the following days. After ash emissions decreased, three incandescent degassing points on the crater floor were seen for several days. On 18 August, Strombolian activity resumed and during the night some incandescent bombs rose above the crater rim. Ash emission the following days prevented observations inside the crater, but no blasts were heard. On 29 August, another 1-day phase of Strombolian activity was followed the next day by ash emissions that marked the end of this eruptive episode.

Ash emissions and another episode of Strombolian activity from the summit craters

A strong episode of black ash emission from Northeast Crater (figure 60) during the late morning of 13 September lasted only a few minutes, sending an ash plume 100 m above the crater rim. Red ash emissions from Bocca Nuova and Northeast Crater continued until about 20 September, but explosions of variable frequency and intensity were heard from both throughout the month. Voragine (Chasm) and Southeast Crater exhibited only weak degassing in September. Poor weather prevented internal crater observations.

On the evening of 2 October explosive Strombolian activity resumed at Northeast Crater from two small vents, aligned NNE-SSW in the lowest portion of the crater floor, ~150 m below the crater rim (figure 60). During observations the next morning, loud vigorous explosions were almost continuous, throwing scoria above the crater rim. A slight decrease in the frequency and energy of the explosions occurred that afternoon, although some incandescent bombs fell on the outer crater slope. Activity continued to decline during the night, and on the morning of 4 October Strombolian explosions were restricted to a single vent ejecting bombs up to a few tens of meters above the crater floor. By the evening of 5 October only incandescent degassing vents were present. During the same period, Bocca Nuova exhibited frequent red-brown ash emissions alternating with normal degassing. Ash emission was occasionally accompanied by incandescent bomb ejection. The ash puffs, more copious on the morning of 3 October, were produced by the same partially collapsed vent that was the site of Strombolian activity in August (BGVN 20:08).

Bombs collected on the crater rim (the first accessible material since the end of the 1991-93 eruption) were geochemically comparable with the 1991-93 lavas. The bombs were porphyritic hawaiite with phenocrysts of plagioclase (~16 volume %), clinopyroxene (~4%), olivine (~1%) and Ti-magnetite microphenocrysts in an intersertal groundmass.

The Istituto Internazionale di Vulcanologia (IIV) report below provides an overview of activity during October. IIV reports generally summarize the temporal evolution of volcanic phenomena during the whole month, skipping some trivial details, and frame the ongoing activity in the context of phenomena over a period of years.

Reports detailing activity during short visits made by visiting volcanologists provide a different perspective on the volcanism. One such report for some days in October was provided by a team led by Open University (OU) volcanologists conducting routine deformation measurements during 9 September-14 October. Short visits to the summit craters on 7, 12, and 14 October were also made by Boris Behncke, with additional observations from Carmelo Monaco and Marcello Bianca (University of Catania), Maria Felicia Monaco (Bari University), and others.

Review of July-September 1995 activity. Strombolian activity resumed at Bocca Nuova on 30 July and in Northeast Crater on 2 August (BGVN 20:08). On 30 July spatter was observed inside Bocca Nuova from a new pit crater on the N part of the crater floor. The activity climaxed on 2 and 3 August, when lava jets rose above the crater rim, then stopped on the night of 4 August. Strombolian explosions during 2-3 August issued from a small vent in the lowest part of the crater. Two more Strombolian episodes occurred on 18 and 29 August. A strong explosion from Northeast Crater on 13 September sent an ash plume 100 m above the rim. Ash emissions from Bocca Nuova and Northeast Crater continued until about 20 September, but explosions were heard throughout the month (BGVN 20:09). The OU team noted light ashfall 2-3 km away in the third week of September, and heavier ashfall 50 m from the Bocca Nuova rim on 27 September.

Overview of October 1995 activity from IIV. After a short period of Strombolian activity at Bocca Nuova and Northeast Crater at the beginning of October, alternating mild Strombolian activity and ash emission characterized their activity for the rest of the month. On 8 October almost continuous rumbling noises (like roaring jets) were heard from both craters. On the morning of 12 October intense ash emissions took place from both craters. Bocca Nuova displayed small short-lived ash puffs (5-7/hour), while from the Northeast Crater a dense ash column rising as high as 900 m developed repeatedly (2/hour). IIV field parties working in the summit area reported that the ash emission were accompanied by falling rock noises. However, successive surveys observed neither juvenile nor lithic blocks on the crater rims.

After 12 October Strombolian activity progressively resumed at Northeast Crater and continued with variable intensity until the end of the month. On 19 October Strombolian activity was relatively vigorous and the scoria ejections, up to few tens of meters from the crater rim, were almost continuous. A survey on 25 October revealed an appreciable decrease of the explosion frequency. Bocca Nuova exhibited intermittent ash emissions after 12 October. As during previous activity, they originated in a depressed area of the NW crater floor. Explosions observed on 19 October were accompanied by ejection of a black (lithic?) block to a few tens of meters above the crater floor, but neither glowing at the vent or ejection of incandescent bombs were observed. After 19 October intermittent ash emission progressively decreased, and in the last week of the month weak Strombolian activity resumed at Bocca Nuova. Significant eruptions on 9 and 14 November will be reported in the next Bulletin.

Deformation measurements. Preliminary results from the OU team indicate little ground deformation since October 1994 over most of the network. Summit levelling showed insignificant movement (-5 mm near the summit, +7 mm on the N flank) apart from the area above the 1991-93 dike, which between the W side of Cisternazza and Belvedere showed a fairly consistent subsidence of 17-24 mm. Preliminary GPS computations suggested a radial expansion about the summit of ~15 mm. Dry-tilt stations showed no large tilts.

Details of 1-7 October activity. Observations from the Northeast Crater rim on the afternoon of 1 October by the OU team revealed two faintly glowing vents, ~3-5 m across, on the crater floor. The following night, bright summit glow was seen from Nicolosi (15 km S), and on the morning of 3 October loud explosions from Northeast Crater were heard from the trail 800 m W, which had been covered with a thin layer of red ash overnight. Explosions were again heard late in the afternoon from ~7 km away, and light ash fell near Monte Corbara (5 km NW). While approaching the crater at 1815 on 3 October, two guides and an Italian TV camera crew returning from the rim warned of bombs falling outside the crater. As the OU team moved towards the high ground behind the crater, a large explosion sent brightly-glowing juvenile bombs just over the rim, rolling toward them. A few seconds later a single bomb ~20 cm across landed 10 m away, 100-200 m from the rim. Similar bomb ejections to smaller distances occurred about every 2 minutes until the team descended at 1845. On 7 October, Behncke noted a dense steam-and-gas plume from Northeast Crater. Most of the plume and occasionally some ash rose from the SSE part of the crater floor; falling stones were frequently heard.

Detonations from within Bocca Nuova heard by the OU team on 1 October were only audible from the rim. One vent on 4 October was explosively exhaling gas, and the other was collapsing, producing brownish ash clouds. Behncke observed small Strombolian explosions from Bocca Nuova on 6 October, but only ash emissions the next day. On the 7 October visit, Behncke observed frequent ash plumes from Bocca Nuova accompanied by rumbling noises and the sound of falling stones; Strombolian explosions were frequent.

The Chasm (La Voragine) quietly emitted fumes on 1 October. On 4 October the OU team climbed into Southeast Crater to the edge of the vents, which emitted gas quietly and not under pressure, apart from one area just below the S rim. On 7 October, Behncke heard small explosions, but no ejections or incandescence were seen after sunset.

Details of 12-14 October activity. Between 0800 and 0900 on 12 October a series of collapses within Northeast Crater generated a thick ash cloud. Pulses of rapidly rising ash plumes resulted in a vertical column 800-1,000 m above the summit. After 0900, a dilute gas plume rose from Northeast Crater while Bocca Nuova sent frequent ash emissions 200-300 m above the summit. When Behncke reached the crater rim shortly after 1230, there were vigorous steam emission and explosions from Northeast Crater.

Behncke saw incandescent spots in the central Northeast Crater floor that gradually increased in number and intensity. Pyroclastic ejections became more frequent and vigorous, and soon the incandescent areas were hidden by gas and dilute ash plumes. The ash plumes first rose slowly to ~100 m above the crater floor, but gradually rose higher and became more heavily ash-laden. About 5 minutes after the onset of ash venting, dense convoluting ash clouds began to rise above the rim. Bomb and ash emission steadily increased. The high-pressure gas emission noise at the beginning of this activity changed to a dull rumbling connected with the ash emission. Short pulses of bomb emissions every 5-10 seconds were followed by a dark ash puff. After ~10 minutes, the ash puffs merged into a continuous column that rose hundreds of meters above the rim. Around 1345 vigorous emissions ejected black ash plumes ~1 km above the summit. Periodic ash emissions from Northeast Crater gradually became less vigorous before ceasing that evening.

On 12 October (0800-0900), the OU team heard detonations from Bocca Nuova, mainly from a vent on the E side of the floor, but the larger vent on the NW side occasionally threw 20-cm-diameter lithic blocks 30-50 m high. Ash emissions seen by Behncke after 1230 occurred every 2-5 minutes from the pit on the NW crater floor. Each emission began with block and/or bomb ejections followed by a dense ash plume. The bombs and blocks rose out of the ~50-m-deep pit but remained ~100 m below the rim, whereas the ash plumes rose 100-500 m above the summit. An open vent in the SE crater floor displayed continuous gas emission with occasional explosions that ejected dense gas clouds.

Shortly after 1700 on 14 October Behncke saw a central glowing vent in Northeast Crater. Vigorous high-pressure gas emission produced a roaring noise, and the plume was almost vapor-free. During the first 30 minutes of the visit, glowing spatter was occasionally ejected from the vent. As degassing increased, numerous incandescent spots became visible, aligned more or less concentrically around the vent. After the first half hour, Strombolian bursts became more vigorous, ejecting bombs ~50 m above the pit. About 10 minutes later, the explosions again intensified, and the crater floor around the vent, which appeared more funnel-shaped, was covered with incandescent bombs. Ejections rose ~100 m above the vent but remained far below the crater rim.

On 2 August 1995 explosive activity resumed at Northeast Crater (NEC) (BGVN 20:08). In August and September the activity was sporadic and low in intensity (BGVN 20:09), but after 2 October a vigorous Strombolian phase was observed (BGVN 20:10). Explosive activity occurred again during 19-22 October.

On 1 November there was vigorous spattering and bubbling of magma in a 15-m-wide pit on the NEC floor. Magma degassing formed large bubbles that burst, throwing spatter to the crater rim. In the following days the activity was discontinuous and less intense.

Lava fountaining episodes, 9-14 November. At 0014 on 9 November there was a sudden increase in volcanic tremor, but bad weather prevented summit observations. Between 0105 (at Trecastagni) and 0110 (at Catania, 30 km SSE) ash and lapilli fallout covered the SE flank (figure 61), eventually reaching as far as Siracusa, 75 km from the vent. The episode lasted only a few minutes and the material on the lower slope amounted to a few tens of grams per square meter, although rare dense lapilli broke some skylights and car windows. Fieldwork the next morning revealed that the NEC eruption produced a lava fountain followed by a strong phreatomagmatic blast. Part of the S rim collapsed inside the NEC and was later ejected. A welded spatter deposit several meters thick mantled the upper slope of the NEC cone and was overlain by a few centimeters of ash and lapilli. The bombs varied from 2-3 m close to the vent, to 25 cm at 2.5 km downwind. Several large accidental lithics (up to 1 m) occurred in the very proximal deposit. A large amount of spatter fell into the crater, raising its floor by several tens of meters. The crater appeared completely sealed, with wide red cracks on the crust of the spatter pile. The total volume of tephra from the 9 November eruption was ~1.5 x 106 m3.

Figure 61. Map of the Etna area showing areas affected by ashfall on 9, 14, and 27 November, and 23 December 1995. Courtesy of IIV.

On 10 November a new lava fountain episode at NEC was observed from Catania around 0400-0530. Pulsating magma jets climbed up to 300 m above the crater rim; some were expelled up to 500 m. An ash-and-lapilli column ascended ~5,000 m and was blown SE. The spatter deposit was limited to the upper part of the volcano and in a narrow strip extending ~3 km SE; little ash fell on the middle slopes. The estimated volume of the pyroclastics was a few tens of thousands of cubic meters.

A third episode took place around 0600 on 14 November, and lasted ~3 hours. Between 0800 and 0900 the paroxysmal phase sent dense black ash columns through a white cloud covering the summit until they reached 5,000 m altitude. During the entire episode a non-continuous sustained eruptive column was observed and each ash puff contributed to a plume bent downwind that reached its buoyancy level at 6-7 km altitude. Ash and lapilli rained on the NE flank down to the coast (figure 61), leaving only a few grams of material per square meter on the middle and lower slopes. The proximal spatter deposits, mapped two days later, partially covered the previous ones on the cone and extended ~2 km NE in a band a few hundred meters wide. Lithic blocks and ash were less abundant than in deposits from the 9 November episode. The crater bottom was sealed by back-fallen welded spatter and was ~50 m below the crater rim, 100 m higher than before 9 November. The total volume of tephra from the 14 November eruptions was ~350,000 m3.

The volcano remained quiet after the 3rd episode. Within NEC, only a few large cracks on the welded spatter crust emitted fumes. Bocca Nuova crater showed a normal continuous degassing; Southeast and Voragine craters continued their steam emission.

Lava fountaining episodes, 22-27 November. Late on 22 November continuous glows were observed at NEC and some bangs were heard on the lower slopes. Beginning around midnight, two hours of fire fountaining and intense red glow was visible from Catania. The lava jets remained fairly low (~100 m above the crater rim) so the proximal spatter deposit mantled only the upper part of the cone, whereas the fine material fell on the SE flank as far as the coast. However, the total volume of the erupted material was limited to a few tens of thousand cubic meters, close to that of the second episode.

After the 22 November episode the vent was closed again by material that fell back into the crater. Three days later some bangs were heard at NEC and glow was observed during the night of 26-27 November. That morning seismic tremor rose suddenly and at 0715 an ash-and-lapilli column rose from the volcano. Cloud cover prevented direct observations. Ash and lapilli were carried by strong winds and fell on a narrow band of the N flank down to its foot (figure 61). Lapilli fallout ended around 1000, but the explosive activity continued for several hours. The thickness of the scoria-fall deposit varied from decimeters close to the vent to ~1 mm at 12 km away. The total tephra volume from this 5th eruptive episode was estimated at 0.4-0.5 x 106 m3.

Fieldwork two days later revealed that the proximal spatter deposits of the 22 and 26 November episodes were thinner than earlier ones. Lithic blocks were less abundant than in the 9 November deposits, but large ballistic scoriaceous bombs were found up to 500 m from the vent. The crater floor was completely sealed by fall-back spatter, but every 40-60 minutes a gas pocket broke the solid crust and a single lava bubble burst. These phenomena were observed for a few more days.

Activity during December. In the first half of December the summit craters were quiet, with continuous steam emissions, except for NEC, which had no open vent. A short explosive phase was reported on the night of 6 December. Poor weather conditions prevented observations until 16 December, when continuous Strombolian activity was seen at a small vent on the crater floor; a cone grew within a few days. The activity was characterized by the bursting of single magma bubbles alternating with degassing jets and spatter lasting from tens of seconds to a few minutes. This intense Strombolian activity continued for several days.

Around 1100 on 23 December strong bangs were heard from skiers on the upper slope. Very soon the bangs became frequent and black ash puffs were observed from NEC. Between 1215 and 1220 the first jet of magma rose above the crater rim, followed shortly by several pulses of magma jets and a large eruptive column. Between 1235 and 1305 the paroxysmal phase occurred, with jets of magma that rose 500-600 m (measured on the video record of the surveillance camera at La Montagnola, 2,700 m elevation on the S flank). Fragments from the top of the jets fed an eruptive column that reached 9.5 km altitude (6.2 km above the summit). Clear weather allowed observation of the column from many places on Sicily, as far as the city of Palermo 190 km away. Abundant ash and lapilli fell on a wide band of the NE flank down to the coast (figure 61). A brownish ash plume was emitted by Voragine during the entire paroxysmal phase of the eruption. Around 1330 the eruption quickly declined, but isolated explosions occurred until the evening. This episode was the most energetic among the six at NEC during November and December 1995.

The proximal deposit mantled the NEC cone with meters of welded spatter. In the W and E saddles between NEC and the Central Cone, spatter formed two thick lava flows a few hundred meters long. The E flow was still active during the night of 23-24 December; downslope movement of fluid material in the core produced continuous collapses of large incandescent blocks at the flow front. Crater modifications included the thick new scoria bank and widening and lowering of the S crater rim. Ballistic clasts had been thrown up to 600 m from the vent and landed as cow-pie bombs up to 2 m in diameter. The distal deposit from the eruptive column was made of scoriaceous bombs and lapilli up to 10-15 km from the vent, and from lapilli and a minor ash up to the shoreline, 22 km away. The bombs were very brittle, flat, and up to 30 cm in diameter at 6 km from the vent (observed while still in the air). The scoria-fall deposit formed a continuous band from the vent to the coast, damaging fruit plantations, vehicles, and buildings. The Messina-Catania freeway had to be cleared of a scoria deposit along a 4-km-long stretch. The deposit thickness along the dispersal axis was 6-7 cm at 6 km, 3-4 cm at 13 km, 3 cm at 16 km along the freeway, and 1-2 cm at 20 km near the coast. The estimated total volume of pyroclastics erupted on 23 December was ~3 x 106 m3.

On the days after 23 December eruption only a few blasts were heard from NEC, but on the nights of 27 and 28 December discontinuous glow was again seen, sometimes similar to those produced by mild Strombolian explosions. No further activity was reported at NEC or the other craters through the end of the year.

Tephra characteristics. Bombs and lapilli erupted during the November-December 1995 episodes are highly vesiculated and show glassy and smooth surfaces. Only in the volcanics erupted on 9 November are both vesicles and surfaces filled by reddish, fine-grained non-juvenile material. Juvenile ash consists of: 1) poorly vesiculated tachylitic (glassy) grains; 2) highly vesiculated clasts with glassy, smooth surfaces, and many Pele's hair and shards in the finer fraction; and 3) loose crystals covered in some cases by a thin film of glass.

Generally rounded grains with variable alteration form the non-juvenile fraction. In the ash fraction of all deposits, juvenile material is always the most abundant (60-100%), and preliminary investigation indicates that it increased with time. The juvenile fraction is ~60% of the 9 November ash, ~80% of the 14 November ash, and ~100% of the ash erupted during the following episodes (23 and 27 November, 23 December). The proportions of different juvenile components also changed during the eruptive sequence.

Scoria erupted during the November-December explosive episodes are, like most of Etna's historical volcanics, porphyritic hawaiites with phenocrysts of plagioclase, clinopyroxene, and olivine, and microphenocrysts of Ti-magnetite in a hyalopilitic groundmass. The scoria are more vesiculated and slightly less porphyritic than those erupted in October 1995. The chemical composition of November-December scoria is rather homogeneous even if the 9 and 14 November material is slightly more differentiated than those erupted after 23 November. Overall, the composition of the November-December volcanics is comparable to those of the Strombolian activity at NEC during the first half of October, and to the products erupted in the first days of the 1991-93 eruption.

Seismicity. Seismicity recorded by the permanent seismic network (12 stations; figure 62), during November-December 1995 was characterized by remarkable phases of increased volcanic tremor amplitude. Earthquake activity stayed at very low levels. A few tens of shocks took place and the only significant episode occurred on 24 December when a minor swarm (6 events; Mmax=3.2) was located near Mt. Maletto (NW slope of the volcano) at a depth of ~15 km.

Figure 62. Map of Etna showing locations of seismic stations, tilt stations, and EDM networks maintained by the Istituto Internazionale di Vulcanologia as of December 1995. Courtesy of IIV.

Since the end of August 1995 volcanic tremor recorded at Pizzi Deneri (PDN: ~2 km from NEC, 2,820 m elevation) and Serra Pizzuta Calvarina (ESP: ~7 km from NEC, 1,590 m elevation) stations has shown an increasing trend. This pattern became more evident in late September, when some increases in tremor amplitude were recorded for durations ranging from tens of minutes to a few hours. The most relevant increases in tremor amplitude occurred on 22-23 September, 2, 3 and 21 October, 9, 10, 14, 22-23, and 27 November, and 23 December. This tremor amplitude pattern correlated with visually observed NEC eruptive activity.

The volcanic tremor spectral amplitude temporal pattern at PDN and ESP stations showed a clear amplitude increase. Spectral amplitude peaks were superimposed on the increased trend and corresponded to the episodes listed above. Dominant peaks in tremor spectra recorded at PDN and ESP stations showed a high-frequency (~3.5 Hz) trend coincident with the high tremor amplitude. Each amplitude increase showed similar characteristics.

Ground deformation. After the end of the 1991-93 eruption deformation was dominated by steady inflation, mostly affecting the W and NE slopes. Positive trends of areal dilatation, cumulating at ~14 ppm, were clearly apparent on the SW and NE flank EDM networks (figure 62) following the 1991-93 eruption, while the S network was characterized by a flat trend of areal dilatation for several years. Both the SW and NE networks followed comparable trends, only differing in the recent sharp positive gradient variation (10 ppm) shown by the latter between August and October.

The shallow bore-hole permanent tilt network (figure 62) indicated a progressive increase (starting by the second half of 1993) in the radial tilt component recorded at the stations on the W flank (MSC: 50 µrad) and on the N flank (MNR: 10 µrad), while the S slope showed no appreciable positive variation until July 1995. The eruptive activity resumed at the summit craters by late July-early August, and the renewed ejection of magma appeared to be strictly related in time to the positive variation of the radial tilt at SPC (~15 µrad) and the sharp increase of areal dilatation in the NE sector. Radial tilt at PDN was affected by a sharp negative variation (35 µrad) at almost the same time.

September EDM survey on the S flank. J. Moss noted that reoccupation of a different S-flank EDM network in September 1995 showed only minor line extension since eruptive activity resumed in August. Significant extensions of lines perpendicular to the Valle del Bove accompanied dike emplacement prior to the 1991-93 eruption. However, the July 1995 survey showed only minor changes since July 1994. Over 80% of the lines measured between those two surveys showed extension, suggesting a pattern of broad edifice inflation. The small strain rates suggest that no magma was intruded into this part of the S rift zone prior to September 1995.

After the sixth eruptive episode at Northeast Crater (NEC) on 23 December 1995 (BGVN 20:11/12), continuous steam emission was observed at the other summit craters in early January. After sunrise on 4 January, ash puffs were observed at Bocca Nuova crater (BN). The abundant black ash emissions were apparently not linked to explosive activity; the frequency of ash puffs ranged between 2 and 5/minute and slowly declined during the afternoon. The next day only a white plume was present. A few small ash puffs observed on 5 and 9 January came from BN and NEC.

In the early morning of 17 January, a strong explosion from NEC ejected lithic material. Intermittent blasts (up to 15 minutes apart) were heard during the day, but no ejections were observed. Fieldwork two days later revealed that Strombolian explosions with ash puffs had occurred from two vents in the NW part of the Bocca Nuova crater floor, in the same place where activity resumed at the end of July 1995 (BGVN 20:08). The Voragine crater produced unusually strong gas blasts, but no sign of eruptive activity was observed. NEC produced a strong explosion at 1010, but then remained quiet without any gas emission. A 20 January explosion at NEC had similar characteristics. Explosive activity on 21 January was more intense and caused ash emission mainly from BN, but some strong blasts also came from NEC during the day.

Seventh eruptive episode. During the night of 24 January, red glows were intermittently observed at NEC, and after 0600 on 25 January lava jets inside a dense ash cloud were observed by the surveillance video camera at La Montagnola (3.5 km from the summit). This seventh eruptive episode, 33 days after the start of the previous one, probably began around 0130 when a strong increase in tremor amplitude was recorded by the summit stations of the IIV seismic network. A pulsating ash column developed around 0430 and was flattened down to the ground by strong winds. The lava jets were fairly low (~100 m above the crater rim) so the spatter deposit mantled only the upper part of the NEC cone, whereas fine material was blown onto the NE flank. Lapilli fallout ended around 1045, but the explosive activity continued for several hours. The lapilli-fall deposit covered a sector of the volcano >20 km long and 3.5 km wide at 12 km from the vent, where the thickness of the deposit was 1-2 mm. The volume of the pyroclastic material erupted during this episode was estimated at ~500,000 m3.

During the night of 26-27 January several strong blasts from the summit were heard in the nearest villages and strong red glows were sometimes observed at the summit. This activity was marked by short periods of high tremor amplitude. At sunrise two intense ash emissions from NEC were observed by the video surveillance system. Aerial observations revealed that one or more short lava-fountaining episodes occurred at NEC during the night. A hot spatter deposit covered a wide band on the upper SE flank down to 2,500 m elevation; no fine distal deposit was observed. Ash puffs and blasts were observed and heard from BN and NEC in the following days (in particular on the morning of 28 January) up to around 1000 on 30 January when tremor amplitude increased and ashfall was reported by skiers on the S flank. However, these phenomena vanished in the afternoon.

The summit craters remained quiet in early February, showing continuous steam emission sporadically darkened by minor ash. However, tremor amplitude fluctuated above background levels. On 8 February copious ash emitted by BN thinly covered the snow on the S flank.

Eigth eruptive episode. Another fire fountaining episode at NEC began at 2335 on 9 February and ended around 0115 the next day. Pulsating lava jets reached 200 m above the crater rim. Lapilli fallout covered a narrow band (1-3 km wide) from the vent to the shoreline (25 km away) on the SE flank (figure 63). A light ash fallout reached the town of Catania. However, the estimated volume of eruptive products was the estimated volume of eruptive products was < 300,000 m3.

Figure 63. Map of the Etna area showing zones affected by ashfall in November-December 1995, 21 January 1996, and 9 February 1996. Coordinates are UTM. Courtesy of IIV.

Minor eruptive activity continued until 0145-0200 on 10 February. A strong explosion at 1022 ejected a large amount of material from NEC. Several ash puffs occurred during the day at NEC and BN craters. In the late evening the ash emission at BN increased and Strombolian activity resumed at NEC, marked by increased tremor amplitude that decreased again during the night. At dawn on 11 February several ash puffs were observed at BN; this activity decreased during the day but around 1700 the tremor amplitude increased again and strong Strombolian activity resumed at NEC. Eruptive activity continued through 2130 and then dropped.

On 12 February numerous ash puffs were observed at both BN and NEC. At 0030 the following day strong Strombolian activity was observed at NEC by the surveillance camera. The intensity of explosions grew up to 0130 when another sharp tremor amplitude increase was recorded. Strombolian explosions often threw incandescent bombs up to 200 m above the crater at a rate of ~5/minute until 0200. Strombolian activity gradually decreased and after 0300 was seldom observed. At sunrise several black ash puffs were observed at both BN and NEC craters and ash emissions became less frequent at 1100.

Ash puffs were next observed on 14 February, becoming more frequent on 17-18 February and during the morning of 19 February when BN produced almost continuous ash emissions for periods of up to tens of minutes. At sunrise on 21 February the snow was covered by a thin ash layer. At 1757 pulsating red glows were visible above NEC; at 1830 the glow became continuous until sunrise the next day (22 February). Higher intensity glow occurred for up to a few tens of minutes when bomb ejections were recognized.

During 22 February activity was apparently low, with only a few ash puffs from NEC. At 0240 on 23 February red glows resumed at NEC and continued through sunrise. Red glow resumed at 1820, alternating between a few tens of minutes of strong activity and longer periods of reduced activity. The same phenomena occurred the following night but poor visibility prevented good observations.

Good visibility on the night of 24-25 February permitted detailed observations of the Strombolian activity at NEC. It was continuous all night and produced by two vents; the rate of explosions ranged between 1 and 5/minute, and ejecta rose to a maximum of 150 m above the crater. During daytime no evidence of this activity was recognizable from the surveillance camera, but the next night (25-26 February) the two vents were often active simultaneously and their frequency of explosions exceeded 5/minute; moreover, the strong explosions at the start of each higher intensity phase threw bombs up to 300 m above the crater.

Poor weather conditions after the morning of 26 February prevented regular observations. Decreasing tremor amplitude in late February suggested that the period of quasi-continuous Strombolian activity at NEC ended during daylight on 27 February.

Intermittent ash emissions and Strombolian activity from two summit craters

Adverse weather conditions that prevented observation of the summit in late February (BGVN 21:02) continued throughout March. Ash puffs from Bocca Nuova crater (BN) were seen during some clear periods on 1 and 5 March, and on the morning of 6 March several black ash emissions were observed. Between 1200 and 1300 a sequence of ash puffs was produced from Northeast Crater (NEC). At 1530, another dense black ash puff was emitted from BN. At sunset the snow mantle was discontinuously covered by a thin ash layer. Ash emissions were again observed during some clearings on 7 March.

On 11 March around 2300 a one-hour long increase in tremor amplitude was recorded at the summit stations. During the afternoon of 12 March the weather improved and after sunset pulsating red glows were observed above NEC by the surveillance camera. Glow produced by the Strombolian activity after 1730 was almost continuous until changing to pulses at 1840 and disappearing at 2100. At the climax, red tracks of volcanic bombs were recognizable up to 150 m above the crater rim. The eruptive episode was marked by increased seismic tremor amplitude similar to that of the previous night.

On the morning of 14 March weather conditions became worse and the video link was interrupted. The video link was restored on 21 March and some minor ash emissions were observed. The observations by the video camera remained intermittent due to the poor weather. Around 2000 on 30 March a remarkable increase in low-frequency events and explosion earthquakes was recorded at all stations of the seismic network; poor weather prevented visual confirmation. The phenomena continued until 2100 on 31 March and during the daytime strong pulsing steam emissions, sometimes with ash, were observed at NEC and BN.

Strombolian activity that began the day after the eighth fire fountaining episode (9-10 February) continued in April, building several nested spatter and scoria cones on the NEC floor; these rose as high as the crater rim.

Visiting scientists made observations of eruptive activity during late May and June. The observations revealed continued activity similar to that previously described for this eruptive phase (BGVN 20:06-20:11/12 and 21:02-20:03).

Observations during 26 May-3 June. Activity at the summit craters was described by Marco Fulle following visits on 26 and 30-31 May and on 1 and 3 June.

Bocca Nuova was filled by thick steam on the afternoon of 26 May, but there were many strong explosions. A vent on the SE side of the crater was emitting steam jets. A vent with a lava pond on the crater floor was ejecting meter-sized lava clots 20 m high. When the pond level was close to the bottom, Strombolian explosions rose 50 m. Northeast Crater (NEC) erupted thin ash and black bombs, but later produced Strombolian explosions every 10-50 seconds that sent a few bombs 50 m above the rim directed towards the E. No bombs fell outside the crater.

Observations of NEC beginning in the late afternoon of 30 May were made for 18 hours from Pizzi Deneri and the W rim of NEC. Strombolian explosions ejected black bombs 100-200 m above the rim at 1-40 second intervals. On the morning of 31 May many meter-sized incandescent bombs were ejected well beyond the SW crater rim due to a strong wind. After 30 minutes, this activity changed to predominantly ash eruptions. Eruption intensity soon increased again, ejecting lava clots and dark bombs well above the rim.

For two hours on the evening o f 1 June observers watched from the W rim of Bocca Nuova and Voragine. Bocca Nuova contained two vents with active lava ponds aligned N-S. The N vent produced Strombolian explosions 50 m high every 10-30 minutes. The S vent produced Strombolian explosions 10-30 m high every 5-20 seconds. A third vent in the SE side of the crater produced steam eruptions every 2-10 minutes with red glow during the steam ejections. Voragine produced steam jets when NEC was inactive. During five hours of observations at NEC from the W rim on 3 June, Strombolian explosions every 2-50 seconds rose 100-200 m.

Observations during 1-20 June. While making GPS measurement of a deformation network on the volcano's upper S flanks on 1-20 June, J.L. Moss and co-workers observed summit activity.

During 1-5 June, no explosions or ash clouds were observed, but the summit vents were vigorously steaming. On 6 June, local guides reported explosive activity at NEC. On 9 June steam degassed strongly from the summit craters, and yellowish fumes escaped from Southeast Crater.

On the night of 10 June Bocca Nuova exhibited two strongly glowing vents on the crater floor, each producing mild Strombolian explosions every 5-10 seconds and ejecting material to heights of a few meters. Larger explosions took place about every minute, but no material was ejected above the crater rim. At La Voragine crater (the Chasm), a single glowing vent on the crater floor produced mild, audible, Strombolian explosions every 5-30 seconds that ejected material a few meters high.

At NEC Moss's group felt radiant heat and saw intense heat shimmering above radial fractures around the crater rim. Very strong gas emissions prompted them to wear gas masks. The crater was filled with dark (non-glowing) solidified lava; it formed a fractured dome from which a dense gas/water mixture escaped. No Strombolian activity was observed.

During June eruptive activity was very strong. Two episodes of fire fountaining occurred at Northeast Crater (NEC), the first on 6 June and the second on 25 June.

On 2-3 June there were many Strombolian explosions at NEC. After two days of quiescence, the 9th episode of fire fountaining since November 1995 was recorded at 2015-2110 on 6 June by the seismic network and surveillance video camera. Poor weather conditions prohibited clear observations. Volcanic tremors increased after 0550 on 6 June, with tremor amplitude reaching a maximum at 1944 and returning to normal around 2300. Lapilli and ash dispersed NW up to a few kilometers from the vent (figure 64).

Red glow appeared above NEC around midnight on 7 June. From 0200 to 0400 on 8 June, there were almost continuous strong Strombolian eruptions. Between 2240 on 8 June and 0355 on 9 June, and on the evening of 9 June (2020-2200), the "La Montagnola" surveillance video camera recorded red glow above NEC and weak Strombolian activity inside the crater. On the nights of 10 and 11 June, red glows lasted a few minutes. During 0400-1200 on 12 June, ash explosions took place at NEC at intervals of 10-20 minutes. On the early morning of 13 June Strombolian activity at NEC resumed for ~20 minutes, and during 1755-1900 on 13 June there was almost continuous ash emission. The video camera system was not operational during 14-17 June. Explosions took place at NEC during 2317-2350 on 17 June, and during 0004-0300 on 18 June strong and continuous Strombolian activity was recorded. From 18 to 24 June Strombolian activity was low and sporadic.

At 2347 on 25 June, the 10th episode of fire fountaining began. The paroxysmal phase lasted for 35 minutes (0015 to 0050), and at 0055 on 26 June the episode ended. The fire fountains rose ~300 m, and a column of lapilli and ash reached several kilometers above the summit. The lapilli fell on the NE flank of the volcano (figure 64) and extended as far as Taormina, 28 km from the vent. Ashfall was also observed in Messina, 70 km away. An increase of volcanic tremors with oscillation of amplitude was recorded after 2200 on 24 June. The maximum amplitude of volcanic tremors was reached at 2230 on 25 June and lasted for more than one hour.

After the 10th eruptive episode, NEC remained quiescent until the morning of 28 June. At around 1700 that day, strong ash explosions opened a new vent on the W side of the crater floor that had been sealed by fire-fountain spatter. In the last week of June, Strombolian activity also resumed at Bocca Nuova crater (BN). Several red bombs were thrown up to the crater rim.

Beginning on 2 July, the surveillance camera recorded continuous red glow during all the nights in July, indicating persistent Strombolian activity inside NEC. During the first ten days of July, explosive activity at NEC was relatively low, then gradually increased and became vigorous on the early morning of the 14th. On 16 July Strombolian activity abruptly decreased. The next day several black ash puffs were observed, eruption intensity increased again, and a second vent inside the crater began to erupt. During 18-20 July, Strombolian activity was very strong and the magma level inside the vents was high. Jets of magma rising up to 200 m resulted in the fast growth of two cinder cones.

Two eruptive fissures formed on the W and S flanks of the cone at NEC on the morning of 21 July. The fissure on the W flank (N70°W, 40 m long, ~3,220 m elevation) issued a lava flow that was active until that afternoon and produced a small lava tongue, which was 100 m long, 10 m wide, and 2 m thick. The W lava flow continued until 22 July. A lava flow from the S fissure moved on the flank of NEC that borders the Voragine crater at ~3,230 m elevation.

On the morning of 23 July, lava spilled from an ephemeral vent on the lowest portion of the E rim at NEC (at an elevation of ~3,250 m). The vent drained a crusted lava flow field that had formed in the previous days and covered the E side of the crater floor. The lava overflow traveled E for more than 1 km inside the Valle del Bove, and its front reached an elevation of ~2,650 m, above the hornitos chain from the 1986 eruption. Close to the vent, the lava flow was ~2 m deep and ~1 m wide, but at a few tens of meters away from the vent, it was up to 10 m in width. On the morning of 25 July, a preliminary estimate of the lava output rate was 0.3 m3/s. Direct thermocouple measurements of the lava flow temperature gave a maximum value of 1,080°C. On 28 July the lava issuing from the E side of NEC crater rim stopped, resumed a few hours later, then stopped on the morning of 29 July.

The lava erupting inside the Voragine crater stopped on the morning of 30 July, and resumed a few hours later. On 31 July the lava flow and gas emission continued, and lava filled the degassing vent of the Voragine crater at a rate of ~0.5 m3/s.

On 1 August eruptive activity increased very quickly, and on the night of 3 August three lava flows poured from the NEC crater rim and moved down the E slope of the summit cone into the Valle del Bove. Explosive activity continued at NEC and threw large bombs up to 300 m above the crater rim.

The following summarizes the monthly reports of the Istituto Internazionale di Vulcanologia (IIV) for the period August-7 November 1996.

Activity during August. Strombolian activity continued from two vents in the cinder cone on the W border of the Northeast Crater (NEC) rim. There were almost continuous explosions; bombs and lapilli were thrown up to several tens of meters above the crater rim. Lava flowed from the NEC effusive vent on the S flank down into Voragine crater.

On 3 August an ash emission lasted three hours and three lava flows produced by ephemeral vents on the E rim of NEC moved toward the Valle del Bove. Activity increased on 4 August at 2045 and continued until the morning of 8 August without interruption. During these days explosions were heard in the villages Zafferana Etnea and Nicolosi (15 and 10 km from NEC, respectively). Three to four new vents emerged; they were aligned roughly N-S, produced Strombolian explosions, and modified the shape of the cinder cone from circular to elliptical.

Activity increased again on the morning of 14 August when several blasts from the middle flank of the volcano were heard. Two vents erupted together throwing lapilli and bombs up to 300 m above the NEC. An aa lava flow from the E slope of the cone moved into Valle del Bove, and sudden high lava jets were observed at the vent in front of the Voragine crater.

On 15 August some ash emissions occurred, and red glow was observed continuously during 15 and 16 August. During the night of 16-17 August the Strombolian activity suddenly decreased. On 17 August a few explosions ejected both incandescent and dark material. During the night of 17-18 August the lava flow in Valle del Bove stopped. On 19 August the lava flow in the Voragine also stopped and overall there was no eruptive activity. Poor weather conditions prevented clear observations.

On 21 August from 1040 to 1130 small Strombolian explosions occurred at NEC. On 28 August from 0700 to 1230 several ash explosions coincided with collapse of the cinder cone inside the NEC.

Activity at Bocca Nuova crater was characterized by mild and discontinuous Strombolian eruptions from vents in the N part of the crater floor. The other two summit craters, Southeast Crater and Voragine, produced only steam emissions. Although the degassing vent in the Voragine crater was filled almost continuously by lava from 21 July to 19 August, it remained open and degassing for the whole period.

Activity during September-October. Gas and steam were continuously emitted from the summit craters during September. On 17 September at 2100, weak Strombolian activity was observed at NEC for one hour.

On 1 October at 2100 Strombolian activity resumed at NEC with an explosive sequence. Initially the explosions occurred every 5 minutes; from 2230 the glow of the explosions became continuous and stronger, lasting all through the night. During the daylight of 2 October, puffs of black ash were observed until midday. Bad weather condition prevented observation for some days, and when the summit craters became visible they emitted only gas and steam.

Another short period of Strombolian activity occurred during 13-14 October, beginning around 1200 on 13 October with several small black ash puffs every 10-20 minutes. Weak red glows observed at night on NEC indicated low Strombolian activity. The following morning ash puffs were still emitted at NEC; eventually during the day the ash emissions became less frequent and then disappeared.

On 18 October a single ash explosion occurred and on 20 and 23 October several ash puffs were noted. These explosions opened a large pit crater on the E side of the intracrater cinder cone built during the strong eruptive activity of July and August.

On the N floor of Bocca Nuova (BN) during October, an incandescent hole produced sporadic weak Strombolian activity. The occurrence of ash in the plume after 24 October marked an increase in explosivity. Voragine had no eruptive activity in October; only quiet steam emission and some puffing were observed at the large hole on the lower part of the crater floor.

Activity during 1-7 November. On 2 November some intense red flashes were reported from BN and NEC at 0045; within the next 20 minutes larger explosions produced stronger glows at BN. Strombolian emissions at BN resumed during 0125-0130 and during 0205-0210, but ejecta reamined within the crater and minor emissions continued until 6 November when one ash puff reached 1 km above the summit. The emission came from an old degassing vent located on the inner wall, on the SE side of the crater. The associated explosion enlarged the vent and caused debris from the inner walls to cover a wide portion of the S crater bottom. A few small lava blocks fell on the S crater rim and a brown, totally non-juvenile ash deposit covered the E sector of the summit cone of the volcano down to a few km from the crater.

These events preceeded the resumption of activity at Southeast Crater (SEC), ending five years of complete inactivity (quiet since Autumn 1991, shortly before the beginning of 1991-93 eruption). On 6 November, after the large phreatic explosion at BN, very low Strombolian activity was reported from two vents on the floor of SEC. The close temporal occurrence of these phenomena and the alignment of the Strombolian vent of BN, the phreatic vent of BN, and the Strombolian vent of SEC, suggested that a very shallow magma intrusion moved from BN toward SEC and triggered its Strombolian activity. However, such interplay is normal among the Etna summit craters when the magma level reaches a very shallow depth. During the evening of 6 November some very low glows were observed above the SEC. In the following days SEC eruptive activity remained at very low levels of intensity.

Overflights of the summit craters on 5, 6, and 7 November confirmed that Strombolian activity occurred at BN almost continuously from one or more closely spaced vents on the top of the intracrater cone.

The following summarizes the activity observed at Etna from November 1996 through March 1997.

Activity during November 1996. On 2 November both Bocca Nuova (BN) and Northeast Crater (NEC) produced Strombolian activity lasting all night. On 6 November an old degassing vent on the inner wall on the SE side of the crater emitted one ash puff that reached 1 km in height. The associated explosion enlarged the vent and caused debris from the inner walls to cover a wide portion of the S crater bottom. A few small lava blocks fell on the S crater rim and a brown, totally non-juvenile ash deposit covered the E sector of Etna's summit a few kilometers down from the crater. The same day Southeast Crater (SEC), quiet since Autumn 1991, began very low Strombolian activity from two vents on its floor. During the evening of 6 November glow was seen above the SEC, but in the following days eruptive activity remained very low.

Overflights on 5-7 November confirmed that Strombolian activity occurred at BN almost continuously from one or more closely spaced vents on the intracrater cone. Besides almost continuous ash puffs produced on 10 and 11 November at BN, there were two large ash explosions on 16 and 18 November.

The surveillance video camera captured intervals of night glows over NEC (from 0110 to 0130 on 23 November; from 1812 to 2100 on 24 November; and for a half-hour during the night of 27-28 November). At SEC between 23 and 28 November weak Strombolian activity occurred sporadically. BN produced several discrete Strombolian explosions from 2005 to 2140 on 28 November.

Activity during December 1996. The summit craters erupted infrequently in the first half of December and more frequently in the second half. On 15 December Strombolian activity was reported at both craters during 2340-0200. At NEC episodes of Strombolian activity occurred on 17 December (1730-2140), 18 December (0150-0438, 1830-0045), 19 December (1740-1747, 2125-2215), and 21 December (0400-0540, 1730-1800).

During the morning of 16 December some ash explosions occurred at BN; in the morning of 19 December there was both an ash plume and ash puffs. A small wall collapsed on 21 December at BN's S vent adding particulate to the summit plume.

Between 22 and 24 December some short periods of Strombolian activity were recorded at both NEC and BN. At SEC during December there were no available video records; however, direct observations revealed only very weak and discontinuous activity.

Activity during January 1997. BN's Strombolian activity was reported at multiple times: 1800-2100 on 7 January; 2348-0102 beginning on 9 January; during several ~1-hour intervals, 11-13 January; and as weak, continuous activity, 14-24 January. More episodes occurred at 1940-2130 on 27 January; at 0800- 1400 on 29 January; at 0800-0900 on 30 January; and at 0400-0500 on 31 January.

NEC's Strombolian activity took place at these times: 2000-2100 on 7 January; several ~1-hour episodes between 11-13 January; and sporadic weak episodes, 20-22 January. Later, from 25 January until the end of the month, the eruptive vigor rose; ash emissions occurred during 0800-1400 on 29 January and eruptions could be watched from a distance from sunset until 2200 on 30 January.

On 25 January, at 1920 SEC began one hour of very weak Strombolian activity. The activity of SEC had formed a small cinder cone inside the crater and some short fan-shaped aa lava flows departing from the cone.

Activity during February-5 March 1997. The first half of February was characterized by the continuous emission of ash from BN; bomb and lapilli ejections inside the crater were reported on 11 February. There was strong degassing from NEC but weak emissions from Voragine Crater. Inside SEC the small cinder cone spattered continuously, though weakly. During the second half of February both BN and NEC plumes frequently contained ash.

A field survey on 5 March revealed that some cinder cones formed in BN. One of these cones threw bombs up to the crater rim and a vent in the SE part of the crater gave off strong emissions of gas and red ash. Fresh lava tongues were identified on 5 March at the W edge of the SEC cinder cone. A later image of SEC (11 March) is shown in figure 65. Voragine crater only degassed whereas NEC collapsed slightly as indicated by minor debris in the crater.

Figure 65. Aerial view of SEC on 11 March 1997 showing the intracrater cinder cone and the small lava flow first seen on 5 March. Photo by A. Amantia; provided courtesy of CNR-Istituto Internazionale di Vulcanologia.

A map prepared from observations carried out on 11 April of Voragine and Bocca Nuova craters is presented in figure 66. The position and altitude of the points shown by stars were measured with ranging binoculars (model 1500 DAES; on loan courtesy Leica-France) from two observation points (circles) on the rim of the craters. Photos of the crater interior were also used to draw the map.

Bocca Nuova crater measured ~170 m deep and had two active cones on the crater floor. The N cone, 25 m above the crater floor, was the most active. Its Strombolian activity threw ejecta close to Monumento, a spot on the crater's N rim. The S cone, 35-40 m above the crater floor, appeared composed of two coalescent cones, and was less active then the N one.

The depth of the Voragine crater measured ~150 m. Quiet steam emission was observed escaping from the large hole on the lower part of the crater floor.

During April the mild Strombolian activity of Southeast Crater (SEC) continued at the same level as in previous months (BGVN 22:02). Every night the surveillance video camera at "La Montagnola" recorded episodes of Strombolian activity that lasted from a few minutes to an hour, with some isolated explosions. Direct observations on 11 April revealed that the small SEC cinder cone had changed its shape and was still producing new lava flows from the breaks on its flanks. Strombolian activity occurred also at Bocca Nuova (BN) with spattering as high as the crater rim. On the N side of the crater floor a cone was spattering from one of its numerous vents. On the SE side a new vent opened near an older one; both were strongly degassing and mildly spattering. Strong degassing was observed both at Voragine and Northeast Crater (NEC). In addition, the collapse of NEC's floor was indicated by debris in the crater. Field surveys during the second half of April revealed no variations in the volcanic activity or in the craters' appearance.

During May, Strombolian activity at the N and S vents of BN varied daily in intensity from low-level degassing and minor eruptive activity to magma boiling on the crater floor and almost continuous Strombolian explosions. Volcanic bombs were thrown as high as the crater rim, but none fell out of the crater. SEC continued to produce minor, almost continuous gas explosions and some spattering from the dome-shaped cone. Lava emissions generally lasted for a few hours. This hornito-style activity was eventually interrupted by sudden vigorous explosions caused by temporary blockage of the conduit. Examination of bombs and lava blocks that had fallen beyond the SEC rim confirmed that the magma was more crystal-rich and viscous compared to the scoriaceous material erupted by BN. No explosive activity was reported at NEC except a few small brown ash emissions, probably caused by collapse of the degassing vent's walls. Enlargement of the pit-shaped vent was seen during a survey with a Civil Protection helicopter.

During June the explosive activity gradually increased at both BN and SEC. In particular SEC's hornito-style activity was frequently interrupted by intense explosions, but there was no appreciable variation in lava emission. BN activity remained within the crater boundaries with minor lava flows; eventually bombs thrown out of the crater fell on the cone's upper N slope.

Continued activity from three craters through mid-July; crater descriptions

The following summarizes observations, organized by crater (figure 67), made by Boris Behncke of the activity and morphology of Etna's summit craters during visits on 14 June, 11 July, and 16 July 1997. Additional observations of activity through 18 July are reported.

Figure 67. Sketch map of Etna's summit craters as of July 1997. Locations of eruptive vents and recent lava flows are indicated. Courtesy of Boris Behncke.

Voragine. This crater was degassing from a central pit during visits in October 1995 and September 1996. Lava effusion from nearby Northeast Crater into Voragine in July-August 1996 did not fill the pit. However, during 14 June the pit was obstructed, with only wisps of steam escaping from its E rim. The 1996 lava flows from Northeast Crater had been almost completely removed by collapse. On 13 July the crater reopened. Mountain guides reported ejections of ash and possibly fresh scoria.

Northeast Crater. After the activity of late 1995 to late 1996, Northeast Crater became Etna's highest summit, surpassing the remains of a 1964 cone on the SE rim of Bocca Nuova. The 1995-96 activity and subsequent collapse completely altered the crater, which had a deep pit with vertical walls in early October 1995. The SW part of the crater contained a cluster of small cones and partially overlapping craters; none were active on 14 June. The N part of the crater was occupied by a lava platform which filled the crater in June-July 1996. The W edge of this platform was made of large tilted slabs. A lower platform covered by a lava flow from the cone cluster partially encircled a deep ~100-m-wide pit that was the site of Strombolian activity. Loud roaring from the pit on 14 June preceded emissions of dense yellowish ash-bearing gas plumes at intervals of 1-2 minutes. Activity on 11 July (when viewed from Bocca Nuova) appeared similar; there were no incandescent ejections after sunset.

Bocca Nuova. Since the resumption of magmatic activity in July 1995, two principal eruptive centers have been active in the ~150-m-deep pit: one vent at the base of the SE crater wall, and a group of vents in the NW sector of the crater. The former only emitted gas during the past two years; the latter exhibited periodic Strombolian activity and lava effusion. On 14 June the SE vent had Strombolian explosions every 10-15 minutes, with fragments rising 50-70 m; on 11 July explosions reached the crater rim (>100 m above the vent) and fresh bombs were found to the SE outside of the crater. The NW vent cluster consisted of three boccas aligned NW-SE on 14 June that generated nearly continuous small Strombolian bursts and lava emission from an area to their E. At times the northern vent filled with bubbling lava. On 11 July three vents were aligned E-W; lava effusion occurred from vents to their E or SE.

During a visit on 16 July, a large spatter cone with a crater 20-30 m wide had formed in the NW area of activity, where there had been three small vents only five days earlier. The crater of this new cone was filled with vigorously boiling and spattering lava. Explosions from the SE eruptive vent occurred about every 3-5 minutes, at times ejecting bombs high above the SE rim (~150 m above the vent). Similar activity continued through 18 July.

Southeast Crater (SEC). On 14 June noises characteristic of Strombolian activity were heard ~2 km S of the crater, but no ejections rose above the crater rim. Daily observations from Catania (~30 km S of the summit) began on 7 July, coinciding with a slight intensification of activity from SEC. At night, nearly continuous Strombolian bursts were visible. During the following evenings activity appeared more discontinuous, with periods of activity up to 20 minutes separated by up to several hours. A visit to the crater on the evening of 11 July found that a cinder cone in the N part of SEC had almost risen as high as the crater rim. Strombolian activity, in cycles lasting ~15-20 minutes separated by intervals up to 20 minutes, sent bursts as high as 150 m above the vent. An incandescent lava flow from a vent ~20 m below the cone's summit moved down the S flank of the cone, extending ~200 m to the S base of the inner wall of SEC. Slightly older flows around the active lobe still had incandescent spots. Despite the episodic explosive activity, effusive activity appeared reasonably constant. Night observations from Catania during the following days disclosed continuing explosive activity from SEC.

The floor of Southeast Crater, gradually being filled by a growing cone and lava flows, had risen to within <10 m of a low point on the SE crater rim by 16 July. As of 18 July the cone in SEC's N half was as high as the crater rim (~50-70 m above the lowest part of the crater floor). Lava flows issued more or less continuously from boccas on the upper S and SE flanks of the cone, forming a complex lava field to the S, SE, and E. At night, explosive activity from the cone's summit is visible from Catania.

Strombolian activity and lava overflows; all four summit craters active

The following report, jointly submitted by scientists from the Università di Catania and the Istituto Internazionale di Vulcanologia, expands on information from the first half of July (BGVN 22:07) and adds observations of activity at each crater through the end of the month. A review of the eruption since July 1995 places the current activity into context. Between 19 and 31 July, moderate eruptive activity continued from the summit craters, including occasional small overflows from Southeast Crater (SEC). Magmatic activity began at the Voragine in mid-July, followed by continuous Strombolian activity from a central pit in Northeast Crater (NEC) at the end of July (after about eight months of relative quiet). A visit on 5 August revealed magmatic activity from all four summit craters (figure 68).